I think that one problem in 0ad are team bonuses. The fact that there are very good team bonuses and shoddy ones, affects also the fact that some civilizations are more / less played respect to others. In fact, let's talk about ibers and rome. If we think about it, iber civilization is quite popular exclusively thanks to their team bonus, while rome (that is anyway a good faction) is rare in 1v1 games, while in team games is very used, widely thanks to it's team bonus.
Anyway, I don't agree about the fact that a civilization like ptolemies, britons or gauls should have a better team bonus. These are already the strongest civilization, and a better team bonus would boost all players to use only these three civilizations.
In my opinion better team bonuses must be given to the less used civilization. Let's make some examples:
Athenians: their team bonus is effective only in naval maps (and is not so op anyway), so is it useful considering that naval maps are played really little?
Carthaginians: considering that trade is the best bonus that can be given to this civilization (looking at it's story), why not to increase it? A 10% bonus doesn't motivate anyone to play this civilization even in a trade-based game (Persia has in my opinion a similar problem)
Seleucids: to reduce the cost of new civic centres (that are also a building that is not so used in team games) only of 100 resources is quite unuseful: do 100 resources of each type affect the game?
I know that surely some players already discussed about it, so I would also like to know if there are some ideas about it or a mod that fixes it.