What's stopping you is your own sense of good sportsmanship. You could pre-make your game in any other jam and just re-skin it during the 48 hours.
Still I'm also in favor of a theme, since it can get the creative juices flowing and it's always interesting to see what different people do with it.
So I took the liberty to create a quick poll, I hope the organizers are ok with this! You can vote on themes and suggest your own, so everybody who wants a theme, get voting:
This is obviously unofficial, but maybe if a bunch of people vote, it'll be kinda half-official.
Oh yeah I already added five random themes from https://cjanssen.bitbucket.io/themegen/ but like I said, you can suggest your own, too.