For classes that typically involve external power sources, these are subclasses that are defined by the absence of such entities (not unlike the idea of oathbreaker paladins).
Ideas for common features and themes:
Their power comes from "nothing", maybe it is sentient void (like tartarus) or maybe it is the absence left by the disappearance of another being , or just something that defies explanation
They drain power (such as spell slots) from others rather than draw it from other sources.
Maybe they can open fissures to absorb energy (reduce damage taken)
Gravity and necrotic spells
Briefly sever magical connections preventing others from casting certain spells, perhaps by school or caster alignment (Evocation and divination would be good ones).
Banishment spell
Demiplane/AoE that destroys objects inside it (antimatter arrow style but less broken)
Somehow counter features typical to their class
Impose penalties on spell casting (e.g. -x to spell attacks/DC)
Founded by a character who's name is a John Cage pun. Prevents an bonuses being applied to certain rolls, likely with a use of bardic inspiration. Low level may be preventing advantage or benefits from spells and magic effects, high level might be outright removing all modifiers and just having a straight d20.
"Through the warping of reality or time by powerful and meddling beings, an ancestor of yours now has never existed, yet you remain. A forgotten loose end in reality's attempts to correct itself." Reality warping stuff (e.g. switch locations and effects of creatures, be in multiple places at once, be nowhere or no when and remove conditions on self)
Warlock
Void
Patron be a sentient vacuum, a pit (like Tartarus) or a living demiplane that is hostile to objects and creatures thrown inside. Probably gets some graviturgy and banishment style spells.
For classes that typically involve external power sources, these are subclasses that are defined by the absence of such entities (not unlike the idea of oathbreaker paladins).
Ideas for common features and themes:
Done