So I love D&D, and one thing I like is that the rules change when the game changes are reflected inworld with some form of lore sometimes. In a homebrew setting I've been working on, I want much of the game to function in the 5e ruleset but have eras of time with a simpler more 0e feel to it. Plus the setting itself has some fluff and flavor to spellcasting I want to seek criticism on. Bear with me, this explanation might seem lengthy.
Spellcasters and Spellcasting:
By and large much of the options for spellcasting and spellcasters won't change, but in the settings early eras (which I'm playtesting right now) class options and spell options are heavily limited. Restricted only to the Survivor classes and the NPC classes which players can take freely as though they were full classes, for now we focus on the Spellcaster as it's own class. It would have access to it's own spell list in this setting consisting entirely of all spells originally available in "Men and Magic" the original 0e hand book with minor alterations to adjust the spells as they have been updated over time, and additions to account for spells that have long since fallen out of the game (here's looking at you Sticks to Snakes). Further more, for ALL eras of the game world spellcasters (including wizards, clerics etc) do not use spell slots. Which brings me to the second point.
Mana
Spellcasting in this setting require a caster to manipulate magical energy, or mana. Players who can cast spells as a class ability (not a racial one), will use the optional Spell Points system to cast spells as laid out in the 5e dungeon masters guide. The Spellcaster (as a class) cast spells as though they were a half-caster according to the Spell Points table (though they cannot normally cast 6th level spells). With the exception of the early eras spellcasting tiers are largely unaffected (though 10th level spells exist above 9th tier in later eras that require significant work to cast and can only be cast by 20th+ level casters) for the early era of the setting, Spellcasters can only cast up to 5th level normally* (see below) but are aware of the existence of 6th tier magic and can cast this level of spell in incredibly rare circumstances if they possess the correct implement to do so. Which brings me to my next point.
Mana Stones and Spellcasting Implements
In this setting, all spellcasting implements (be they rod, tome, staff, orb, etc.) are imbedded with rare gemstones called colloquially "Mana Stones" "Focus Stones" or "Resonator Stones". These stones are no different than any number of Gemstones and in fact they are one and the same. Channeling mana in the setting REQUIRES gemstones to be used as both a focus and a reservoir for their casters mana. A caster of any kind from any era is limited in their ability to cast powerful spells by two things, their level, and the value of their focus gem. Normally gemstones come in 6 levels of value (according to the DMG and previous editions sources), 10gp, 50gp, 100gp, 500gp, 1000gp, and 5000gp. Gemstones of varying value are required to cast higher level spells. A 10gp gemstone, like an Agate, used as an Arcane Focus is only capable of harnessing enough mana to cast 1st level spells at best, and even then only some of the time* (See below), a gemstone worth 50gp can cast spell up to 2nd level spells far more reliably. A 100gp stone, like an amethyst can be used to cast spells up to 4th level, and a 500gp stone like a Topaz is required from 5th and 6th level casting. A sapphire or emerald or the like is necessary for 7th and 8th level spellcasting and any 5000gp stone like a ruby, is required to cast any 9th level spell. Mana stones do not replace other spellcasting implements but must me affixed to them if you are using one. If using a 10gp stone this changes nothing, but any value of stone higher creates a magic item that requires attunement, the rarity escalates with the value of the stone (50gp = common, 100gp = uncommon etc.) but does not create a "+1" implement. IF the gemstones affixed to an implement or being used as an arcane focus is destroyed and he caster has no alternative implement or crystal the caster can no longer cast spell until a suitable replacement stone can be acquired.
[To Clarify: A magic item created by setting a Mana Stone to another Arcane Focus can only be attuned to by a caster capable of casting spells of that stones level. I.e. a caster must be capable of casting 9th level spells to attune to a Ruby (5000gp) Mana Stone implement {Legendary}, but must only need be able to cast 1st level spells to attune to an Onyx (50gp) Mana Stone implement {Common}. Even though an Onyx Mana Stone can allow for casting of spell up to 2nd level, the caster cannot cast 2nd level spell with it if they did not know spell of that level normally, except via Overcasting.]
Overcasting
A mechanic tied to Mana stones and spellcasting as a whole is the concept that each stone contains mana itself. A caster can, once per day, attempt to cast a more powerful spell from their implement than they normally could, assuming they have one that functions in such a manner (such as only having access to 2nd level spells but also knowing magic missile which can be upcast). If they do they expend all their remaining mana to attempt to cast a more powerful spell, even if they have no more mana left to spend. The caster rolls a d% with a chance for the spell to fail based on the tier of magic attempted to be cast x10 (so attempting to cast a Tier 3 magic missile with a 50gp Onyx as your Arcane Focus/Mana Stone [Max spell level to cast 2nd] has a 30% chance to fail.) If the spell fails, the caster accrues 1 level of exhaustion. THEN if the stone could have feasibly cast a spell of that level the stone remains intact. IF it could NOT feasibly have cast a spell of that level (such as the example above) the stone is destroyed, though whatever it was set in is not (if it was set in a wand or other object, without a stone the wand etc. returns to being a normal item rather than a magic item). The only exception to these rules are 10gp stones. 10gp stones are used PRIMARILY to cast cantrips and dweomers*, but skilled casters CAN use them to cast 1st level spells via the Overcasting mechanic. Such spells have a 90% success rate and cost mana (spell points) as normal, and are not limited to being used once per day in this case. Still, if the spell fails the stone is likely to be destroyed. Spellcasters in later eras would rather begin with 50gp gems if possible, though 10gp gems are "usable". Early eras casters have no such luxuries.
[To Clarify: Once per day (1/day) a caster using a Mana Stone (gemstone) as an Arcane Implement or as part of an Arcane Implement can attempt to a:) upcast a spell they know that can be upcast by one (1) level to a level they could not normally cast, or b:) cast one additional spell of a level they know but do not have enough mana (Spell points) left to cast. That player then spends the requisite amount of spell points (for the former) or all remaining spell points (for the latter) and roll a d%. If the result is less than or equal to the level of the spell attempting to be cast x10, the spell fails, AND if the spell that was attempted was beyond the normal limitations of the Mana Stone in question, the stone is destroyed. In either case (that of success or failure to cast) the caster gains 1 level of exhaustion after resolution.]
End Goal
Overall the idea is tied to in world lore around gemstones having magical powers, but the end goal is also to make spellcasting more challenging and more resource dependent, providing the feel that casting magic is taxing on a caster and requires thought and set up as well as a proper cost where upgrading ones abilities is concerned. In addition, magic is a powerful tool in many campaigns and my goal isn't to hamstring it, but mitigate it a bit. High level magic is supposed to be VERY rare and very taxing on a caster in the setting, even in later more advanced eras but practically unheard of in times where spells like Chain Lightning draw as much shock and awe and reverence as Wish does in higher tier games.
So I love D&D, and one thing I like is that the rules change when the game changes are reflected inworld with some form of lore sometimes. In a homebrew setting I've been working on, I want much of the game to function in the 5e ruleset but have eras of time with a simpler more 0e feel to it. Plus the setting itself has some fluff and flavor to spellcasting I want to seek criticism on. Bear with me, this explanation might seem lengthy.
Spellcasters and Spellcasting:
By and large much of the options for spellcasting and spellcasters won't change, but in the settings early eras (which I'm playtesting right now) class options and spell options are heavily limited. Restricted only to the Survivor classes and the NPC classes which players can take freely as though they were full classes, for now we focus on the Spellcaster as it's own class. It would have access to it's own spell list in this setting consisting entirely of all spells originally available in "Men and Magic" the original 0e hand book with minor alterations to adjust the spells as they have been updated over time, and additions to account for spells that have long since fallen out of the game (here's looking at you Sticks to Snakes). Further more, for ALL eras of the game world spellcasters (including wizards, clerics etc) do not use spell slots. Which brings me to the second point.
Mana
Spellcasting in this setting require a caster to manipulate magical energy, or mana. Players who can cast spells as a class ability (not a racial one), will use the optional Spell Points system to cast spells as laid out in the 5e dungeon masters guide. The Spellcaster (as a class) cast spells as though they were a half-caster according to the Spell Points table (though they cannot normally cast 6th level spells). With the exception of the early eras spellcasting tiers are largely unaffected (though 10th level spells exist above 9th tier in later eras that require significant work to cast and can only be cast by 20th+ level casters) for the early era of the setting, Spellcasters can only cast up to 5th level normally* (see below) but are aware of the existence of 6th tier magic and can cast this level of spell in incredibly rare circumstances if they possess the correct implement to do so. Which brings me to my next point.
Mana Stones and Spellcasting Implements
In this setting, all spellcasting implements (be they rod, tome, staff, orb, etc.) are imbedded with rare gemstones called colloquially "Mana Stones" "Focus Stones" or "Resonator Stones". These stones are no different than any number of Gemstones and in fact they are one and the same. Channeling mana in the setting REQUIRES gemstones to be used as both a focus and a reservoir for their casters mana. A caster of any kind from any era is limited in their ability to cast powerful spells by two things, their level, and the value of their focus gem. Normally gemstones come in 6 levels of value (according to the DMG and previous editions sources), 10gp, 50gp, 100gp, 500gp, 1000gp, and 5000gp. Gemstones of varying value are required to cast higher level spells. A 10gp gemstone, like an Agate, used as an Arcane Focus is only capable of harnessing enough mana to cast 1st level spells at best, and even then only some of the time* (See below), a gemstone worth 50gp can cast spell up to 2nd level spells far more reliably. A 100gp stone, like an amethyst can be used to cast spells up to 4th level, and a 500gp stone like a Topaz is required from 5th and 6th level casting. A sapphire or emerald or the like is necessary for 7th and 8th level spellcasting and any 5000gp stone like a ruby, is required to cast any 9th level spell. Mana stones do not replace other spellcasting implements but must me affixed to them if you are using one. If using a 10gp stone this changes nothing, but any value of stone higher creates a magic item that requires attunement, the rarity escalates with the value of the stone (50gp = common, 100gp = uncommon etc.) but does not create a "+1" implement. IF the gemstones affixed to an implement or being used as an arcane focus is destroyed and he caster has no alternative implement or crystal the caster can no longer cast spell until a suitable replacement stone can be acquired.
[To Clarify: A magic item created by setting a Mana Stone to another Arcane Focus can only be attuned to by a caster capable of casting spells of that stones level. I.e. a caster must be capable of casting 9th level spells to attune to a Ruby (5000gp) Mana Stone implement {Legendary}, but must only need be able to cast 1st level spells to attune to an Onyx (50gp) Mana Stone implement {Common}. Even though an Onyx Mana Stone can allow for casting of spell up to 2nd level, the caster cannot cast 2nd level spell with it if they did not know spell of that level normally, except via Overcasting.]
Overcasting
A mechanic tied to Mana stones and spellcasting as a whole is the concept that each stone contains mana itself. A caster can, once per day, attempt to cast a more powerful spell from their implement than they normally could, assuming they have one that functions in such a manner (such as only having access to 2nd level spells but also knowing magic missile which can be upcast). If they do they expend all their remaining mana to attempt to cast a more powerful spell, even if they have no more mana left to spend. The caster rolls a d% with a chance for the spell to fail based on the tier of magic attempted to be cast x10 (so attempting to cast a Tier 3 magic missile with a 50gp Onyx as your Arcane Focus/Mana Stone [Max spell level to cast 2nd] has a 30% chance to fail.) If the spell fails, the caster accrues 1 level of exhaustion. THEN if the stone could have feasibly cast a spell of that level the stone remains intact. IF it could NOT feasibly have cast a spell of that level (such as the example above) the stone is destroyed, though whatever it was set in is not (if it was set in a wand or other object, without a stone the wand etc. returns to being a normal item rather than a magic item). The only exception to these rules are 10gp stones. 10gp stones are used PRIMARILY to cast cantrips and dweomers*, but skilled casters CAN use them to cast 1st level spells via the Overcasting mechanic. Such spells have a 90% success rate and cost mana (spell points) as normal, and are not limited to being used once per day in this case. Still, if the spell fails the stone is likely to be destroyed. Spellcasters in later eras would rather begin with 50gp gems if possible, though 10gp gems are "usable". Early eras casters have no such luxuries.
[To Clarify: Once per day (1/day) a caster using a Mana Stone (gemstone) as an Arcane Implement or as part of an Arcane Implement can attempt to a:) upcast a spell they know that can be upcast by one (1) level to a level they could not normally cast, or b:) cast one additional spell of a level they know but do not have enough mana (Spell points) left to cast. That player then spends the requisite amount of spell points (for the former) or all remaining spell points (for the latter) and roll a d%. If the result is less than or equal to the level of the spell attempting to be cast x10, the spell fails, AND if the spell that was attempted was beyond the normal limitations of the Mana Stone in question, the stone is destroyed. In either case (that of success or failure to cast) the caster gains 1 level of exhaustion after resolution.]
End Goal
Overall the idea is tied to in world lore around gemstones having magical powers, but the end goal is also to make spellcasting more challenging and more resource dependent, providing the feel that casting magic is taxing on a caster and requires thought and set up as well as a proper cost where upgrading ones abilities is concerned. In addition, magic is a powerful tool in many campaigns and my goal isn't to hamstring it, but mitigate it a bit. High level magic is supposed to be VERY rare and very taxing on a caster in the setting, even in later more advanced eras but practically unheard of in times where spells like Chain Lightning draw as much shock and awe and reverence as Wish does in higher tier games.