EB currently does not use cultures, Dumbass. Sorry, I couldn't help myself.

So here's the situation. It seems we have 5 cultures that are likely to be used. Two more to reflect the hundred remaining cultures of Europe, Asia, and Africa. The Germans, I don't think, were much of a threat to the status quo prior to the end of the Roman Republic, and the chances of them imposing their culture on the neighbouring Celts. Likewise, I don't think Africans and Arabians were in a position to create a large empire as this time, like conquering and Africanizing Carthage for example.
Here's what I think will be the cultural makeups of certain regions...
Italian Peninsula - Celts in the northern regions, perhaps with a small Roman minority (5%) to reflect traders etc. This would also give the player, and hopefully, AI an incentive to bring these minorities into the 'rightful' faction.
Romans would represent a majority, perhaps with a miscellaneous culture to reflect the Samnites, Campanians, etc which in time would become Romanized. In the south and in Sicily, would be a Greek majority, say 90% Greek and the other 10% being Roman and Phoenician colonists.

Anatolia - Greek and Persian majority (that is, if the Greeks did not assimilate the Persians by 272BC). Some 30% Celt population in Thrace, some 80% in Galatia. Also, there could be a misc. culture to reflect the mountainous peoples who escaped Greek or Persian assimilation?

Bactria - mostly steppe nomads and Persians with a Greek minority. This would compel that faction to recruit fewer Greek units and rely more on native troops

Egypt - this one will be tricky. Obviously, the Greeks were in charge at this time, but what about the rest of the country? Sure, there were likely still some Persians, but what about the Africans, the Egyptians etc. Should they be all listed under Other?

As it stands, all cultures unfortunately cannot be represented so I think it would be best to focus on the cultures that had a likely chance at expanding into large empires