I was thinking about the way the dwarven faction works as when you start the game, you select a different sub-faction, and that could possibly be used for the remaining two after Imladris/Gobbos are finished in the main mod.
I've never played the game before 4.0 (I hated the BFME2 logic) so I have no idea..
Men of the east, with Harad and Rhun being the main sub-factions to select with khandish being a large buildplot option
This idea IMO is a must... I have thought of it for too long and I suppose that the Easterlings should be given their chance, since they are presented in both the books and the movies, what about the corsairs being their subfaction?
For the rest of your post I don't know how the factions will have depth and numerous troops to be presented because of the absence of their presentation in both the movies and the books, (especially in the movies)
Do you have any ideas concerning heroes, powers etc?
Last but not least, what would you say about a dunlending faction?