A game leading to a given position is called a proof game. Of course any proof game can be prepended with 1.Nf3 Nf6 2.Ng1 Ng8, so we defined a shortest proof game as, duh, the shortest possible proof game. Especially interesting are unique shortest proof games. For example, 1.e4 e5 2.Nf3 is not unique - White could swap moves. In contrast, 1.e4 e5 2.Bc4 is unique.
Rules of the fun game: First one creating a position without a unique shortest proof game loses. And that's it.
Do you have ideas for strategies or even a forced win for one side? Checking might be a good idea to force the hand of the opponent; 1.e4 d5?! 2.Bb5+ and now Nc6 or c6 lose, but anything to d7 is OK. (EDIT: And don't let your opponent capture, since one-side noncommuting moves run out quite fast.)
(For testers: Proof game checkers exist.)