As many as possible multiple leaders for each civ should be the motto, every person is different, so were great leaders.
We should be able to play against aggressive and rational leaders from same AI civ opponent, preferably in a single game, on succession-type based game terms.
Players will likely settle for a single leader yet another time, however, like with civilizations and Uniques in the game, the more the better, and the more of each included in the game the more money FIRAXIS and Y2K will make.
Make no mistake about it, and I`m sure they fully realize this.
The only problem I think is tech (hardware), making the game available to as many potential customers as possible out there.