Keep the light on: Explore a maze but your torch keeps running out, find torch refills. This was actually my first LD back in the day then the theme was:
Explore: Find collectibles in a proc gen forest using different methods of traversing.
Castle: Rampart x tower defence.
Keep it contained: Lemmings-style game but keep the ones who know THE THING from leaving.
Roots: Forest sim, grow roots to find water and resources.
Treasure hunt: Deduction game/puzzle/exploration: you get a map with weird symbols and clues and have to find the treasure in a large(ish) terrain.
Piracy: Bring a pirated copy of Sid Meier's Pirates to a pirate with a pirated time machine.
Sacrifice: You're an evil cult leader but also a vegan and have to figure out substitutes for the sacrifices in rituals.
Garden: Stealth/heist game where you steal veggies from a garden as a bunny/mole/whatever.
Single use: Dating game, but if you use the same pick up line twice and the datees find out, you're screwed.
Power source: NPC gives you awesome magic powers, but also goes faster than your walk and slower than your run.
It spreads: Use mouse to spread butter on a bread without spilling. The shapes of breads get increasingly more ridiculous.
Strength in numbers: Strategy/puzzle: your troops in each tile double every turn, but if the capacity is reached you lose. They can spread to adjacent tiles, but only if the amount of troops is divisible by the # of free tiles.
Strange machines: Explore a strange world of machines and figure out what they like to lure them to their enclosure.
Mutation: Simple arcade game (e.g. shooter) but every power-up changes the controls randomly.
Summoning: Deduction/exploration game where you need to find out demons' favourite foods to summon them.