Here are my ideas for each theme, in case any of them seem interesting and you haven't got many ideas, or are just curious of what other people are planning:

--Summoning--

Summon enemies to fight for you, and all you can do is summon said enemies, like an advanced TABS game. However, killed enemies drop mana points, so it is your goal to try to collect them to summon more allies while also trying not to get caught in the fight. The only thing damage dealt to you does is reduce your mana, but if you reach 0 you die, so it is best to keep a bit of mana with you at all times to avoid death.

You can also control the AI of your summoned minions.

--It Spreads--

Platformer which enforces speed in order to try to escape the corruption.

--Recycle--

Weapons have durability. However, when they get destroyed, they can still be used to craft better weapons.

--Tunnels--

Rage game where you race through tunnels with hard controls. Probably only mouse or spacebar.

--Strange Machines--

Fighting/Puzzle platformer where you fight and solve puzzles with strange machines. Your machines get increasingly more conditional, and it is up to you to be able to complete the game by using your machines to your advantage.

--Single Use--

Fighting games with single use attacks, all of them do different things, and are interesting to use. (I dont really give much of an explanation here but its probably my favorite idea)

--Strength In Numbers--

Same thing as the summoning game but less revolved around single units and more around using tons of units to defeat one large one.

--Fishing--

Fishing game, where you have to throw your hook as far down as possible, and the lower you get the better fish you can find.

Survival raft-like game but 2d.

--Push Your Luck--

A fighting game which rewards risky behavior, probably giving you abilities when your low on health or making preexisting ones more powerful (I hate games which involve actual luck in any way).

--Digging--

Mining game with fighting where you are also a mage who can use the gems to fight. Also you don't mine with a pickaxe you mine with a staff. You can use your environment to your advantage, there will be gravel, lava, explosive minerals, which can all be used to destroy your enemies. Your final goal is to reach the bottom of the cave. Where you will fight against a final bossfight. I might just make a cool liero like game instead though, idk.

--Cartography--

Dungeon crawler with cartography, when you start the game a map is chosen randomly based on a seed. Your goal is to map out whatever you can then learn.

--Pick Your Poison--

Dungeon crawler but each room you choose an additional defect but also a bonus. You fight your way through more and more rooms with general buffs but also more and more very specific defects.

--Decay--

Someone has cursed you and as long as he is alive you will decay, you must defeat him while you are still strong in order to win the game.

--Collapse--

Parkour with a collapsing building.

--Mutation--

Gain random perks and use them to fight

It would be interesting to see some feedback on these

These are the themes I have decided to suggest. I always like themes that can be interpreted in many ways, with those more relevant to gameplay than to the setting being the ones I prefer the most, which is what influenced my suggestions.