how i do that is by trying to make a huge list of ideas i want to do, then narrowing it down bit by bit, and even if you can't narrow it down to one just combine multiple of them for a really wacky idea. make sure the idea is:
A: Fun/Creative(if its another generic platformer then its not fun)
B: A fun theme(does not have to follow game jam theme)
C: Not too challenging to do(only 2 weeks boi)