Most likely they will add 2 factions in an add-on, it would be more profitable to release a couple of add-ons and start working on the next part of the game so that it doesn't get too repetitive. They could cram the remaining factions into these add-ons... although they might also drag it out and make us wait :(
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal