I really like the idea of a North-African nomad faction, but I doubt it is possible to implement such a faction due to cultural limits. Other than that, I'd go for a Pergamum, Syracuse, Yuezhi (I know, they probably won't make it) and perhaps a faction like Rhodos-Halicarnassos to spice things up in Asia Minor. I have to admit I don't know whether there is any historical ground to implement the latter faction.

I'm not against Belgae either, but they don't rank high on my wish-to-see list.