Could we make "multiplayer only" games ??
If allowed should it be Online or local or both ??
As far as we know we could do anything (except Offensive content or AI generated stuff) so you can even do local multiplayer only.
Even though I personally don't think that's a good idea as a local multiplayer game is hard to play for single individuals. (unless you add bots, which is a great idea both for local and online as you can't expect many people playing your game at the same time during a jam.)