originally i'd vote for a numidian, pergamon and britain based factions but they are already in.
so from a game-experience pov my prefferances for the other factions are rather similar to the great majority of posters from 2007 onwards (i actually read the first 9 pages before i noticed how old this discussion was

)
1. boii - as said before their presence in central europe and north italy can serve a great deal in slowing german and roman expansion patterns, especially in the gaul theater.
2.a second iberian or celto-iberian faction to duplicate with more accuracy (or at least provide role playing) the political situation there. i.e. if some tribes can be used by either side of the roman-cathaginian conflict as allies, this would not automaticly include all of the iberian peninsula. and 2 factions might even present a more durable buffer against romano-cathage expansion north then 1.
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