My preference goes to use the roman slots for the Gaulish factions, with an unified Rome. The Gauls were seperated in a few major tribes led by high kings, who ruled over all the other tribes. The Arveni, the Aedui, the Veneti and the Belgae spring to mind.

Another similar solution is to carry the romans over to Greece, were they take the place of Athens, Sparta, Corinth, and Epirus. This makes Greece the powerhouse it was, though divided and tiny.

In both plans I'd advise to start the civil war immidiately, or to let the Senate be taken out as fast as possible.