Its a cool feature but i think they are going to concentrate on the diversity of the different factions instead of sub-factions.
But if they would implent it ,it would be awesome to see subfactions in this kind of way.
- Dark elves ( prefer to see it as a faction alone)
Elves - Wood elves
- High Elves
- Men of the North ( some kind of viking like humans)
Human - Human empire
- Horse lords ( army with good cavalry bonusses.
Just an example but there would be just a change in skins and you get a additional bonus with resources + 1 or 2 extra units