I don't think there should be a third X rep. Give me something from classic, Legends, or Battle Network. I'd like to see TvC Roll, Dr. Wily, Protoman, Tron Bonne, Megaman.exe or Bass.exe.
I'd be okay with a revamped Rock Volnutt too.
Though we'll probably get Zero.
I gotta disagree here, though. As much as I love TvC!Roll, and would rather have the MMZ!Zero over his X-series counterpart, I actually think keeping a fairly "unified" front by using just the X series as the main/only MM series in a game like this, makes it so that you can do a better job with maintaining a certain "aesthetic" when it comes to representing that particular brand. If this game is possibly your first exposure to Mega Man at all, then using X, Sigma (and Zero) is certainly a fine choice as any, imo.
After all, at least one Capcom rep went on record to say that one of the MM brand's main difficulty was that there was a lot of subseries that they were all trying to run concurrently, at one point. Starting small and being a good steward with one segment of the brand almost sounds like a sound strategy, in comparison!