I know the concept of a blue mage in D&D is bound to get crazy/unbalanced, but I had fun making this and wanted other people to give it a try or give feedback. (Apologies if the format is messed up. It's a copy+paste from a Google Doc, which you can find here Blue Mage Class Homebrew

Core Blue Mage Traits

Level Progression

Blue Mage Class Features

Spellcasting 

As a blue mage, you have limited spellcasting. You can cast spells from the Blue Mage spell list. 

Trophies: As a blue mage, your spellcasting is channeled through observed or defeated opponents. The piece or essence might manifest in the form of trophies like a creature's hair, horns, teeth, limbs, etc, and act as your spellcasting focus.

Cantrips

You know two Blue Mage cantrips of your choice. Whenever you finish a Long Rest, you can replace one of your cantrips from this feature with another Blue Mage Cantrip of your choice. When you reach Blue Mage levels 10 and 14, you learn another Blue Mage cantrip of your choice, as shown in the Cantrips column of the Blue Mage table.  

Spell Slots

The Blue Mage Features table shows how many spell slots you have to cast your level 1+ spells. You regain all expended spell slots when you finish a Short Rest. 

Prepared Spells of Level 1+

You prepare a list of level 1+ spells that are available for you to cast with this feature. To start, choose two level 1 Blue Mage spells. Identify & Shield are recommended. The number of spells on your list increases as you gain Blue Mage Levels, as shown in the prepared spells column of the Blue Mage Features Table. Whenever that number increases, choose additional Blue Mage spells until the number of spells matches the number on the table. The chosen spells must be of a level for which you have spell slots.

Spellcasting Ability

Intelligence is your spellcasting ability for Blue Mage Spells

Azure Memory (Level 1)

When a creature with a CR equal to or lower than your blue mage level dies within 60 feet of you, you can absorb a piece of its essence. As a bonus action, you learn one of the following:

- A creature trait (e.g, Darkvision, Amphibious) [This does not include a creature's speed] 

- A natural weapon attack (e.g., a breath weapon, tentacle, claw, bite, etc.)  

- A limited-use ability (e.g., Etherealness, Displacement, etc.)  

This essence becomes a “Learned Trait.” You have a limited number of traits known (see table above). At the start of each day, you prepare a number of "active traits" (see table above). You may swap out traits during a long rest. Active traits can be used a number of times per day equal to 2x your proficiency bonus plus your Intelligence modifier (minimum 1). Your use of active traits replenishes after a long rest. Unless a trait specifies casting time, activating the trait requires an action. If the trait does not specify duration, the trait lasts for 1 hour or until dispelled.  

*Flavor Note*: The active traits may appear as eidolons, or physical transformations; however, the point of origin for these learned traits is always the caster, unless modified by the Mystic Savants' summoner feature. 

Attack rolls, damage rolls, and saving throws you trigger are calculated using your intelligence modifier. 

If you try to exceed the number of learned traits for this feature, you may choose a learned trait to "forget"; that trait becomes inaccessible until you learn it again from another creature.  

Monster Lore (Level 2)

You may double your proficiency bonus in nature or arcana when studying a creature. As a study action, you can take 1 minute to identify a creature’s traits,  resistances, or vulnerabilities with a successful intelligence check (DC = 10 + CR). 

Mimicry Archetypes (Level 3)

• Monstrous Shifter: Focuses on racial traits, transformations, and resistances.
• Azure Hunter: Can absorb martial abilities, use weapon techniques, and reactive skills
• Mystic Savant: Specializes in absorbing magical traits or casting enemy spells once absorbed.   

Ability Score Improvement (Level 4)

You gain the Ability Score Improvement feat or another feat of your choice for which you qualify. You gain this feature again at Blue Mage Levels 8, 12, and 16. 

Cobalt Recovery (Level 5)

Once on each of your turns, if you have no uses of “Active Traits” left, you can give yourself one use by expending a spell slot (no action required). In addition, you can expend one use of your “active traits” (no action required) to give yourself a level 1 spell slot, but you cannot do so again until you finish a long rest. 

Perfect Recall (Level 18)

You have a library of learned traits up to 20. When using Azure Memory, you can take 2 traits from a fallen enemy instead of 1. 

Epic Boon(Level 19)

You gain an Epic Boon feat or another feat of your choice for which you qualify. Boon or Recovery is recommended 

Master Mimicry (Level 20)

Once per long rest, you can temporarily gain all abilities of a single creature you have absorbed for 1 minute, including attacks, movement speed, and resistances.

Subclasses

Monstrous Shifter

Monstrous Physiology (Level 3)

A monstrous shifter focuses on racial traits, transformations, and resistances of their targeted quarry. Starting at level 3, you may choose one racial trait you already know; this trait no longer counts against your active traits for the day. You gain one additional racial trait to use in this way at levels 10 and 14.  Additionally, you can switch out your active traits during a short rest.  

Ferocious Attacker (Level 6)

Your natural weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Adaptable (Level 10) 

When a creature attacks, you can use a reaction to give yourself resistance to that damage type. You have resistance to that damage type until the start of your next turn. 

Apex State (Level 14)

You have mastered numerous monstrous physiologies, allowing you to tailor your traits into a “Apex State”. You expend one use of active traits to enter this state. While in the Apex state, you gain the following benefits

+2 AC

Advantage on strength & dexterity saving throws against attacks you can see. 

A bonus to attack and damage rolls equal to your constitution modifier (unless negative) 

Once you use this feature, you cannot use it again until you have taken a long rest. 

This state lasts for 1 hour unless dispelled or you become incapacitated. 

Azure Hunter

Martial Prowess (Level 3)

An Azure Hunter studies the martial prowess of other creatures to use against them. Starting at level 3, you gain proficiency in martial weapons and medium armor. Your Azure Memory is also more refined, allowing you to mimic the weapons and techniques of other creatures as learned traits. When you see a creature take the attack action, you may, as a reaction, learn that attack.  When attacking with a weapon or learned trait, you add your intelligence modifier to the attack and damage rolls. 

Azure Smite ( Level 6)

When you make a melee weapon attack, as a bonus action, you can choose to empower it with monstrous energy by expending a spell slot. The attack deals an extra 2d8 damage at level 1 and an additional 1d8 per spell slot level above 1. You may choose the damage type.

Invoke Ability (Level 10)

At the start of initiative, the Azure Hunter may choose to place themselves immediately after a hostile creature. When using this ability, that hostile creature makes a Wisdom saving throw; on a failed save, the creature must spend its turn attacking the azure hunter at disadvantage. On a success, the creature spends its turn as normal.  The Azure Hunter can use this ability a number of times equal to its proficiency bonus. The Azure Hunter may only use this ability once per combat unless the original hostile creature dies; they can then spend an action to use this ability again.

Arcane Combatant (Level 14) 

You have seamlessly integrated your martial and magical abilities. When dealing damage with an active trait, you may use an additional active trait that deals damage as part of that attack action. (the active trait is expended as normal.) 

Mystic Savant

Enhanced Spellcasting (Level 3)

Your understanding of spellcasting goes beyond the traditional Blue Mage. Your spellcasting table is as follows. When a creature capable of spellcasting dies, you may learn one of its spells as a learned trait. This does not count as a prepared spell, and can be of a spell level beyond what you can cast. 

Echo Spell (Level 6)

When you see a hostile creature cast a spell, you can temporarily hold it in your mind to be cast on your next turn. Casting a spell in this way requires the use of an active trait, and the spell must be no higher than half your blue mage level, rounded down. If the spell requires concentration, the spell ends at the start of your next turn. 

 I Know That Trick (Level 10)

You gain a plus 1 bonus to saving throws imposed by hostile creatures. This bonus increases to plus 3 if you have fought the creature before.

Summoner (Level 14)

You are able to bring the essence of a slain creature onto the battlefield. When you use an active trait to attack a creature, you may instead choose to summon the creature to use their trait instead. You can summon the creature up to 60 ft from you on an unoccupied space that you can see, and the creature disappears at the start of your next turn. The summon has hitpoints equal to three times your blue mage level. If a target leaves the summoned creature's threatened area, you may use your reaction to have the summoned creature take an opportunity attack. 

Blue Mage Spell List