Another idea: I think each civilization should start with no unique traits, but develop them in the Ancient/Classical period, based on what you produce, where your gold goes, etc. So to be a financial leader, you build a lot of cottages to start with to earn the x1.5 economy bonus. And to become a spiritual leader, you focus your efforts on religious buildings. The benefits of an agressive leader are earned by building an army early on, and the benefits of a protective leader are earned through producing archers and building walls. And the more valuable traits (Philosophical and industrial, in particular) are harder to get. Therefore, it would be possible to be a Philosophical/Industrial leader, but it would difficult to achieve and would involve shifting focus away from military production to achieve, weakening you to attack. So it still balances out.
I'm with this a notion but I don't think it should be limited to ancient/classical age; I have I think argued before for leader traits changing regularly (ideally once an Age in a game with seven or more distinct Ages) to reflect whatever your priorities are, and also possibly how well you are doing in any given direction of development compared to the other civilisations in that game,