I mean you didn't expecify the game theme or type, are you gonna tell us the same day it starts? If not what stop us from starting right now our mockup? 

Themes of jams are usually announced at start of submissions.

I was thinking actually to have no theme, at most I might suggest optional theme at the start of the jam to get ideas.

In the end this jam focus more on the fun factor, if you have an idea you might plan ahead.