I remind you that this is a game mod not a historical project.. so don't forget that the gameplay is most important... With losing playable barbarian factions we lose a lot, and playing a rebel campaign is the hardest thing in RTW (believe me... I tried it).
So why don't we instead make the factions represent one major tribe from them... Example: Gaul > Aedui or Arverni , The Britons could be the Iceni or Brigantes, Germania would be the Suebi, GCS > Sparta, Iberia > Lusitanians , as for Numidia they were a kingdom. We would give them 1-2 settlements, and the other towns will be rebel each acting as a different tribe.This is what EB made. Also, this opens the possibility of using a big map.