Sub Factions

All previous points considered, I would opt for a Reinforcement Deck 2. Simply because it would eliminate any confusion that any other combo/name might. The box would clearly state that it was an expansion for variety. A way of rejuvenating your favorite faction that might grow a bit stale with over use. The sales trick is to make it seem desirable enough to buy without being so desirable that you woulod be a fool to own only the basic and expansion decks.

Which is...

Another argument for pdf only versions initially. They could, over time, go from play-test, to beta, to tournament legal, all at almost no cost to YMG, as it could be player diven. This also would be an excellent venue for trying out homebrewed factions, again, if they were popular enough, YMG could publish them as their next expansion.

My only concern with this would be that with so many "designers" having input on each faction it would undoubtedly get to a point where no one was satisfied. I would suggest an "Originator" title where whomever started the faction was given final word and control, everyone else could only "suggest." The Originator could voluntarily pass the title to someone else if they felt estranged from what their original idea had become, or were just tired of it for whatever reason.