I'm thinking neutrality should have to be pre-approved. This is literally a battle where nobody can afford not to choose a side. Even if someone is powerful enough for Rumplestiltskin to be wary of, he's going to either bring them to his side or eliminate them. So nobody's exempt from the wrath of at least one side. Even if they're not willing to cooperate with the heroes per se, surely they can at least subscribe to "the enemy of my enemy is my friend" and fight a common foe if not becoming all buddy-buddy. The alternative could be forming a third faction that is actively competing with Rumplestiltskin and wishes to take him down, but also to conquer the kingdoms of the good folk as well. Sideline characters in this game just don't seem to make that much sense. There could even be an edict from the untouchable magical realm that should someone not respond to the call to battle, they'll suffer consequences, perhaps the loss of their abilities. Whether they be good or bad, as long as they stand on the front lines and contribute to restoring the balance of power (as both sides are sure to have casualties and in the end will likely return the status quo as a result). Kind of like the Force; there's always Sith along with Jedi, so destroying all evil just can't happen.