Gameplay-wise, another German and Iberian faction (probably Celtiberians) would be needed. I mean, the Sweboz have too little trouble both in the beginning and in the later game. Oh, and IIRC, guys from the "German Team" informed us in the "Sweboz slightly underpowered" thread that they wanted to add the another German faction. In case of the Celtiberians - an another faction in Iberia would create a situation similar to Aedui-Arverni thing...

Numidia would also be good (more trouble in Africa is always good, as it's rather empty in almost every TW game or mod), and its inclusion seems quite likely, seeing as it always was a some kind of "a faction that the team wanted to include but couldn't (because of the faction limit)".

Greece is already cluttered and the only thing that it needs is balancing, and it's a never-ending process.

Syracuse would be a "one-city faction" that would stop Karthadastim, SPQR and (sometimes) Epeiros for a short period of time and would be eliminated very early in the game, so it probably should stay as the Eleutheroi city (maybe with an unique unit or two).

Another faction (or factions) in Asia would be good and the more trouble the Seleucids have, the better. Maybe the Galatians? Or the Pergamon?

Oh, and the Getai are passive, but maybe some competition would liven them up?