Ideally, there would be some flavor to each civ in addition to a UU or UB, that might come in an aptitude to acquire or discover a particular tech, or a herding civ like the mongols getting a bonus (extra hammer?) from special resources of that type, while the seafearing vikings would get more from sea resources and the agrarian egyptians get more from crops. It should feel a little bit different to play as or against each each civ, independent of leader traits, which would still include a favorite civic.
Ideally, with expansions, each civ would have two leaders to choose from, even if both Chinese leaders were isolationists, both Spanish leaders were religious zealots, and both Americans were expansionists, etc.