Armor of Spell Strength

This armor has a base AC of half of your spellcasting score + your dex mod + con mod (max: 3). The armor then has extra features depending on your class:

Warlock - your Eldritch Blast has a +1 to attack rolls. When your pact item is active, this changes to a +3, and you get a +2 to your AC. If you have Pact of the Chain, your familiar gains +1 to AC and attack rolls. If you have Pact of the Blade, your pact weapon gains +3 to attack rolls. If you have Pact of the Tome, all spells in your tome gain +1 to attack rolls and spell save DC.

Wizard - Whenever you concentrate on a spell, or an effect of a spell you cast is active, you get +1 AC until it ends. An effect of the spell must be directly from the spell, and not caused by it. Whenever you cast a spell from your school, you gain +1 AC until the end of your next turn. At the beginning of your turn, you lose x hp, where x is the bonus AC gained from this feature (minimum of 0). The max AC you can get from this feature is +4

Druid - While in wild shape, you may cast any spell you could in your normal form at the moment. Whenever you cast a spell with material components, you don't need those components, but instead you may lose xd4 hp, where x is the total cost of the components in GP divided by 10. You also have +2 AC in wild shape, and a bonus +1 after a turn of using only the abilities of the form you are in. This +1 bonus clears at the beginning of your next turn.

Sorcerer - You learn one more Metamagic while wearing this armor, and you gain 5 sorcery points. Your maximum also increases by 5. When using your Metamagic, you may expend 2 sorcery points to use a different Metamagic (you still need to pay the cost for the second Metamagic). Whenever you use your Metamagic, you gain +2 AC until the end of your next turn. 

Bard - When you roll for your bardic inspiration or song of rest, add 1d10 to the result. You can use countercharm as a reaction. While under the effect of countercharm, your allies gain +2 AC. Your allies can also expend their Bardic Inspiration to gain +1d6 AC for the next minute. This does not allow the 1d10 mentioned earlier to be rolled.

Notes: Warlock, Wizard, Druid, Sorcerer or Bard.