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:

http://www.rkursem.com/poll/view.php?id=65bfa8163c78fcf43

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.