My choices:

  • An indian faction would be an interesting interesting change
  • Thrakia? Don't know much about Thrace in this period but they were divided in tribes AFAIK, so one of the thracian tribes would be good... a bit too many factions in that area however :P
  • Helvetii, they already have a nice list of regional units


Honorable mentions: Galatia; another germanic tribe (but it would be too similar to the sweboz i fear); a greek city somewhere else (like Syracuse or Massalia)