I'm going with Raw Latex and Daymare Cat or Nightlights (but I haven't watched/played more than like 4-6 of each list and am not going to spend all day watching through them).
As for themes; I'm good for a few weird ones:
Planar Gravity as a core element. Example of an existing game that uses it a lot:
What do babies dream about when they're too young to remember?
- 3.90 / 5.00
- Ages 13+
Interlooping; a.k.a. the interaction of forward and reverse time. I've never seen anything use this effectively, but I've pondered the concept a lot. If you in forward time, kill someone in reverse time, everything they did before that (in forward time) is undone, but they don't appear to die, because they weren't dead after (before that in backwards time) that. If someone in reverse time gives someone in forward time something, in forward time it appears that before that point, the item didn't exist and after that point, both of those people had a copy. Too trippy? Ok.
To Boldly Go Where No Roguelike has gone before. Make games with two Roguelike elements (procedural generation and permanent death); but cross with genres you have either never or very rarely seen those elements in before. Procedurally generated Doomlike? ...Metroidvania? ...Sonic Clone? Golf Game?
More thematic, less mechanic:
Crossover Arcade. Two stage GameJam. Stage one: make a game. Stage two: Everyone releases the characters they made in the Jam to everyone else. Everyone tries to make crossovers between as many of the characters as possible.
You are the Horde. (Play as something that is more often just a Mook; Zombies, Mushroom men, whatever.)
...and I'm running dry for now.