1. the province that Nervii starts in (Germania Minor I think.) has 3 Celtic faction but a starting culture of something like 70-80% German, I know it's a border region but that seems excessive.
2. The AI's calculation on trade could use some work, It seems to me no good reason why two faction that doesn't have much conflict of interest don't trade, at current it seems that if your small no one wants to trade with you, but why would that be a factor? enriching you is no threat to them, I could see two large empires not willing to enrich each other but why should Rome care if some far away tribe gets a little richer? especially if it doesn't even border them. That AI factions very rarely trade with each other (especially over seas) is a huge advantage to the player and disadvantage to the AI IMHO.
(also, to stimulate AI trade, most civilized faction should know each other already off the bat, Selucids probably don't need to "discover" Carthage and Rome. for example. This gives the Civilized factions AI a bigger advantage but I think that's ok. because the AI for good reasons won't send out ships to open up the map like the players do.)
3. I'm not entirely sure how diplomacy can be modded, but I figure distance between the factions should matter, i.e if two factions border or could be reasonably reached by ship they probably would have different calculation then if say a faction that's on the other side of the map.