Not in any particular order, but factions I'd like to see added include the following.

Starting factions:
Mauryan Empire
Belgae
Bosporan Kingdom
Pergamon/Attalids
Chatti
Numidians
Meroe/Ethiopia/Axum

Emerging factions:
Hasmonean Kingdom (or Jewish rebels)
Yuezhi
Mauretanians
Various rebel factions (Roman rebels), perhaps based on political factions (populares vs optimates)