My issue with this jam isn't so much a lack of ideas, as it's having too many ideas and there's no chance for me to make them all happen.
So I'm gonna throw these out there to make it more interesting and hopefully reduce the amount of generic platformers:
Transport Tycoon where the water is sky and there's so much of it the whole map is islands. You get fun shit like blimps and cloudships or whatever, but trains and trucks are cool on the bigger islands.
A Worms-type artillery game where the islands are physics objects and you can toss them around with bazookas or ropes or whatever, split them in two, weigh them down and make them sink, that kinda shit.
You're an airplane with a hook attached to it, islands are once again physics objects and you try to use your rope to pull them together to form a runway to land on, the challenge of the game is not to crash.
You know how everything you place down in The Incredible Machine kinda floats in midair? With the right artpass, you can do a floating islands variation on the concept.
Your game has nothing to do with the theme but you pull a Cave Story. Expect an avalanche of theme points.
Make Bioshock Infinite 2. No questions asked.