SOMEONE has to start it!
Well, with Moldy Crow taking off all bets as far as the EU, it boils down to Balance, Marketable and Reasonable.
At the Star Wars event, Hothie/Doug asked about a possible third, "pirate" faction. The Devs put their collective foot down pretty hard and said the game was Rebellion vs Empire. I'm not too sure they got the idea, though, and I don't think "pirate" is the best descriptor. Correct me if I'm wrong, but the whole notion is of a NEUTRAL faction that could be used either independantly or as addititions to either Rebel or Imperial. This could mean not only the addition of "neutral" fighters like the Z-95 or 'pirate fighter,' but also retconning ships like the YT-1300 and Firespray to work for either side - WITHIN THE LIMITS OF THEIR CREW. That is, Han Solo or Chewbacca will never fight for the Empire, and Boba Fett will never fight for the Rebels. But not every Firespray is piloted by an evil bounty hunter.
I actually thought the introduction of a Neutral faction was going to be the 'game-changer' FFG hinted at - but it wasn't so big as that.
All that in mind, assuming FFG DOESN'T wrap their arms around a Neutral faction, here's my toss:
YT-2400 & Z-95 Headhunter vs. Skipray blastboat & TIE Defender.
That's weighted towards the Imperials, however. A possible Evil alternative to the Skipray could be Hound's Tooth.