Realistically I'm probably not going to ever finish a full class, but it's a little fun to imagine the potential of the below.
Arcanist
Int based full caster
Access to all spell lists (maybe limited by school)
Everyman
Cha based martial/expert
Versatile but weak class with a bunch of features to choose from, roughly equivalent to other low level class features (e.g. fighting styles, CDs, expertise)
Subclasses (Profession)
Impressionist (Multiple personas with different skills)
TBD (Feat/ASI at every sc lv)
Magician (Third-caster, choice of spell list)
Utilitarian (free skill/tool/instrument prof at each sc lv)
Can temporarily edit the shapes of objects to change their properties (eg. edit a sword to be sharper or duller, or make it heavier so it loses the finesse/light property).
Could just be wizard subclass instead (e.g. Geometromancy)
Subclasses (Specialism)
Edge (Can alter shapes of weapons to change dmg, dmg type and other properties)
Plane (Can alter environment, such as making stairs into ramps)
Polyhedral (Meta, Can change number of faces on a die e.g. d4->d6 and vice versa. d20 exempt or high level only)
Witch
Int/Wis based caster
Could be a warlock subclass instead?
Subclasses (Coven)
Coven of Brewers (Makes potions and poisons)
Coven of Familiars (Familiar based features)
Coven of Hexers (Hex/Curse based features, like hexblade but caster based)
Coven of Seers (Prophecy based features)
Coven of Ritualists (Can cast ritual spells from any list)
Has marks that grant features similar to invocations
Mix of ranger and some form of anti-warlock. i.e. they hunt particular kinds of beings and have features well-equipped for that (e.g. Primordial Obstructor has elemental resistance stuff)
Subclasses (Brand)
Brand of the Arcane Obstructor. Anti spellcaster stuff (e.g. hit a creature affected by a spell to harm the creature concentrating on said spell)
Gets animal-like mutations (similar to simic hybrid but at class level)
Invocation-like mutations, such as claws, thick hide etc.
Subclasses
Burrower. Burrowing speed, dark vision etc.
Marine. Water breathing and swim speed
Winged. Glider wings upgraded to flight wings at higher levels
Climber.
Phereromatic
Alt Subclasses:
Aberrant?. Psychic stuff (tentacles will likely remain as flavour)
Amphibian/Aquatic. Utility/Martial?. Swim speed, water breathing, adv on and against grapple (maybe tentacle attacks)
Arthropod.Utility/Control. Climb/burrow speed, chitinous hide, webs, stingers, extra limbs (for spider climb or TWF)
Avian. Versatile. Wings (low level glide, high level flight)
Beast. Martial/Utility.Claws etc. (may use multiple animal groups to chose from, e.g. cannis/ursine/feline. not unlike totem barbarian)
Pack/Herd. Support/Martial. Features for helping, and getting help from, allies. e.g. pack tactics or bonus action help, AC boost to adjacent creatures
Plant. Tank. Barkskin, photosynthesis (extra healing in direct sunlight), bonuses from not moving on turn
Skinchanger / Druid (Spellless)
Martial/Utility version of the druid, without spells
Gets stronger wild shape (maybe some subclasses can do more than beasts). Probably gets extra attack (that doesn't stack with multiattack)
Maybe it'll work differently where, rather than using another statblock, you get a bunch of traits/features but still use your own stats (and HP) otherwise
Subclasses
Mutant. Can turn into certain monstrosities
Gardener. Can turn into certain plants
Hive. Support, helps allies. Maybe can turn into swarms.
Face. Disguise/trickery stuff
Fiend. Can turn into certain fiends
Wildmaster. Even stronger wild shape (but limited to beasts)
Student/Mimicker
Int-based half-caster (not artificer level)
Focuses on mimicking others. Can copy features to some degree (e.g. someone attacks near them, they can mimic a similar attack)
Runemaster
Int-based half-caster
enchants self and weapons to improve martial abilities (slightly more martial artificer)
Some kind of of indiana jones/lara croft style archeologist (with spells)
Warrior with some arcane abilities (slightly better caster than eldritch knight). Needs to be distinct from paladin
Trickster
cha based expert, focuses on deception
can assume multiple personas, each with different skills
Might get a sneak attack feature that's based more on movement and conditions than damage
ability to convince enemy that you are their ally for one turn
Unlikely Allies. You can attempt to smooth talk your way into unlikely and short-lived friendships. As an action, choose a creature you can see and that can understand you. The creature must make a Wisdom saving throw or become charmed by you until the end of your next turn, or until you deal damage to it. While the creature is charmed, it considers you an ally, even if you were hostile only moments before. For example, you might convince a previously hostile creature that you are betraying your actual allies to join them.
Backstab. When you attack a creature that considers you an ally, or is charmed by you, the attack has advantage. On a hit, the hit becomes a critical hit.