they cant really add more factions, as the game will become a even bigger balance mess than it already is.
Well, if you take a look at Dawn of War 2, that one had 6 factions to play after the second addon, and there you actually had all factions being playable against each and mirror games as well. Balance was very much improvable, but it was better than in CoH2 right now imho.
Dawn of War 1 had even more I think, but I haven't played that game so I don't know how it was there.
Just posting it so you see that Relic has done more factions in the past so it is very possible it will happen again.