1) Due to the enormous work load of creating new factions (for me at least) it's highly unlikely that I/we would ever get this far...
2) A much more needed split is the splitting of Rohan from Gondor.
Initally when I was working on SEE myself, I just wanted Rohan as a mini faction of Gondor. That quickly grew into like 40% of the entire faction, and today, we have enough assets to easily fill a Rohan faction. There are a few reasons that I'd like to see Rohan and Gondor split, like in BFME1, and that's far more important than splitting Rhûn faction. (The dominant faction (in terms of architecture) decides the faction name).
3) So, if we manage to reach Beta 4.8 release with a separated Rohan, we could consider doing the same with Harad for Beta 4.9, as indeed we'd have enough units and structures to fill that faction too, and Rhûn faction (with Harad) is going to be huge indeed. Just like Gondor is today...
Side note: Splitting further than THIS, like splitting Rivendell from Lothlórien is not going to happen though. We could go on forever splitting elves into Lothlórien, Rivendell and Mirkwood, just like dwarves could be split, and goblins, and even Mordor into Dol Guldur, Cirith Ungol, Morannon, Morgul... Yeah we could have like 15-20 factions I'm sure =P The line has to be drawn somewhere, and for me it goes with Rohan/Gondor and possibly Harad/Rhun. Dwarves and Elves had a pretty low impact on screen time in the movies so they'll have to share the space EA gave them, and the fascination of the south and east regions that made me do this 7th faction are primarily cause of all the "civilian" assets in game, as well as my personal interest in those far away cultures =)
So, if the modding gods are kind to us, maybe the day will come in a distant future