If the brilliant developers can balance a large number of factions, then of course, they can add 10 more, I think gamers will only benefit from more factions, more variety, and more tactics...
But unfortunately, this is unrealistic, as everyone knows, perfect balance only exists in Starcraft; there are no other truly balanced games, let alone genius ones. The developers can't balance 8 factions, so it's scary to imagine what would happen if they added even more... Although, they don't need to rush; there are no plans for a sixth installment yet, so they can continue to work on add-ons, but this time with a greater focus on balance.