Here are my ideas (so far) for each theme. I have at least one for every theme except Folklore. I'm still not entirely sure what to do with that one. If Folklore wins, I'll probably just use ideas for another theme and apply folklore creatures to it.

Feel free to use any of these, since they're just basic ideas, and even if multiple people were to use them, the execution would be very different.

When I make my game, I'll probably end up combining multiple ideas, maybe even from different themes (but obviously focusing mainly on the actual theme).

Shelter

• Get to a shelter before night
• Defend a shelter from waves of enemies

Break the cycle

• Complete the same set of challenges in a loop, but keep certain abilities from your previous run

Underground

• Survival game where you must live only in resources found underground

Strange machines

• An interdimensional portal that brings monsters into the human world
• Collect strange machines and use them for combat

Paths

• Choose from multiple branching paths, each of which has different pros and cons

Signal

• Follow the signal - if you lose track of it, it's game over

Overgrown

• The enemies are plants that became sentient and turned into monsters
• The player is gigantic
• The enemies are gigantic

Sanctuary

• Basically the same as "Shelter", so any Shelter ideas would apply to Santuary as well

Salvage

• Survive only from remains
• Craft from parts of broken down machines
• Use scraps to keep a fire lit

Reflect

You have no weapons made for offense, you can only defeat enemies by reflecting their attacks * Enemies can transform into a clone of the player in their current state

Locks and keys

• I would go with the same ideas a "Paths", but require a key for each path, adding an extra choice of whether to go back for another key or choose an inferior path

Keep the light on

• Use items you find (around the world? from enemy drops?) to create electricity and keep a light powered - if it runs out, you lose

Tiny Creatures

• You play as a tiny creature in a world that looks gigantic, and must defend yourself against insects who are the same size as you

Limited space

• The playable area continously shrinks and you must hurry before it eclipses the exit

Absorb

• You regain health and/or stamina by defeating enemies
• You absorb the power of plants