Start with nothing - something inspired by Survival Kids, you're on a deserted island, you have to survive, etc

Unconventional travel - you're a piece of bread and you have to escape the kitchen using nothing but the unexplained large number of toasters sprinkled throughout the level

Side effects - an rpg where every monster you kill increases the hostility of the monster village

Small character, big enemies - you play as a knife and fork and you have to defeat food by stabbing it and cutting it up into pieces (larger pieces can still move and shoot at you???) bonus points for number of subdivisions you make

Factory - you play as a yellow crayon that fell in love with a blue crayon and has to navigate the factory without being caught to find them

Shelter - acid rain on an alien planet.

One item, many uses - skull on a stick.

Night is coming - during the day, you water your crops, and during the night, you have to fend off (alien) pests

Making connections - make your way through crowds and confusing ever-changing announcements at the airport to get on your next plane before it departs

Isolation - you're a dog and your owner has vanished and you have to survive in the abandoned house

Duplication - you have a gun, a single bullet, a bullet-duplicator, and a bullet-duplicator-duplicator.

Mutation - you play as a hideous, freakish, science-experiment-gone-wrong transfer student who just wants to be treated like everybody else.

Low Tech - any of these ideas, in monochrome.

Take one, leave the rest - I like @ogelgames 's idea so I'm stealing it. "Collect tons of power-ups, but only take one to fight the final boss."

Evolve to survive - this is by far my least favorite theme as it's almost a specific game. maybe are a lizard of some kind and you get to change one body part in one direction or another (bigger, smaller) every level to deal with the changing environs.