Right now spell slots are game mechanics, they have no lore and don't exist in world.  In another post I made here I had an idea for a gun slinger reskin of Sorcerers.  As a side effect of that idea I turned Spell Slots into ammunition for them.  Now I'm thinking it would be cool to have all Spell Slots to be items for that player.  My idea is that Arcane caster uses crystals while Divine casters use wax seals of varying grades that determine the level of spell that they can cast.  Now matter how you implement it, the lore would be that no caster can ever conjure enough energy to cast a spell in a reasonable amount of time.  They need to trickle charge a battery of some type.  Once it's charged then they can cast spells with all the energy they had stored.  We are taking a game mechanic and were imbedding it in the world, it has a in universe explanation now.  I could also see you doing this treatment for only arcane or divine casting.  Maybe Arcane works normal but divine casting uses seals, or visa versa.  The cool thing about this is that now Spell slots can be shared between the party that are the same type of caster.  That's my I am making both Divine and Arcane magic use different items.  Only Arcane or Divine casters can share spell slots, a Wizard can't give his spell slots to a Cleric, but he can give it to a sorcerer.  They also can be looted from the world now.  As a GM you can now replenish just the spell slots without resetting the parties health, or class features.  It can even be the bases for a whole campaign.,

Here are a few ways I can see it implemented in a campaign.

Extreme Low Magic

In this idea their was once an ancient civilization that used to make Crystals, Seals, or both for casting spells.  This civilization fell apart and the method to create them has been lost.  In this campaign every crystal or Seal can only be used once, so when you cast a spell with a spell slot you need to find a replacement item to cast another spell.  Were completely throwing away what the PHB says about spell slots, you have only what you have found in the world.  They are very much a loot item here.  The party might need to search old ruins, and dungeons in the hopes of finding new items to be able to cast spells again.  Since this is a low magic I assume were not going to be facing magical enemies, and part of the idea of this setting is that magic is rare and really powerful.  The goal of the party is to find the capital of the ancient civilization in the hopes of discovering the secret of making new crystals / seals and bringing magic back to the world.

Low to Medium Magic Setting

I can see several different ways of doing this but most I think of involve traveling a world with known but somewhat rare magic casters.  Were throwing away the PHB again but now each item can be recharged as needed.  If you get a third level spell slot you can reuse it multiple times after recharging it.  I could see doing a Pokémon like game where the parties goal is defeating gym leaders to get more spell slots for the caster.  Maybe there's also a larger plot at play where there trying to stop a team rocket like organization for gathering crystals for themselves.  Maybe we have a trials of Hercules type deal where your a plucky band of adventurers trying to make admen's for past wrongs by powering up a crystal that just so happens to be powering up the parties caster.   Another way to run it is that your a divine caster that needs to banish Demons or devils to get more seals.  The more powerful the demon or devil the higher level the spell slot.  That's where my mind goes

High Magic

Were back to PHB spell slots.  You get what the book says you get because the character summons the crystal / seal as needed.  You can do any normal D&D campaign with this but now you can give spell slots as loot, as well as they can share them.  That's the only difference.

What do you think.

I think it would take a lot of rebalancing to make them work in this way, but I can see any of these being a really fun, interesting setting for a custom campaign.

To be fair, "spell slots" are no more ridiculous a game abstraction than "MP" is.  Both are ways to measure how much magical power a person can produce/safely channel in a given period of time.  Spell slots only feel weird and game-y because D&D is the only thing that uses them; if spells ran off of MP instead the way damage runs off of HP, nobody would bat an eye.

That said?  As Transmorpher noted, the idea could be the basis for some interesting custom campaigns, especially for those DMs who're frustrated at how prevalent magic is in 5e.  In conjunction with either the Slowed Healing or Gritty Realism rules, one could turn the game into an ultralethal death slog that requires significant, exacting preparation and zealous resource management, which is absolutely in line with what some groups are looking for.

If you really want to be extra-punishing and ensure nobody ever plays a caster ever, ye could instead require that casters find/loot or create magical scrolls, at the usual costs.  Heh, your 'spells known' is your recipe list for scrolls you know how to create, but otherwise every magical effect in your game comes only when read off an enchanted parchment.  Admittedly, at that point you are severely shafting your spellcasters  since their basic and most defining class feature now costs significant gold and downtime to use, so you may want to tweak spell numbers or CR such that what spells they do get to cast have an equally resounding impact.

Or just let your spellcasters eat shit and enjoy FighterParty 2020.  Heh, either way really.

They tried it in AD&D 2e in book “Players Options: Spells &Magic” (if I remember the title correctly) published by TSR and it had a lot of balance issues. It might be worth a look to see what they came up with.