Interesting idea. Hmmm... I think I'd prefer donations before the jam than 50% creation after, because having to work with "what's there" is the super-unique part of the jam. There have been other jams where you have to "use these X sprites for sure", but then you HAVE to use those sprites instead of choosing what you want.
It might be interesting (to me) though if the spritesheet was ONLY donated sprites from say 32 folks, and they just provided a cart with eight sprites, and I put them together JUST before we all get to see what they are! That would still get the unique flavor of the jam, but no one knows fully what will be in the final cart, including me! And I like that only X sprites roll over to the next one (so it's not just an ever-increasing number).
Now, I was overdramatic above -- I did the original most of the original sprites, but my bud Toby did all of spritesheet two and others donated sprites as well. So I get to play with plenty of sprites I didn't make! :D My main task this jam was making the "unfinished game", heh.
Thanks for the input!