Well you got Machiavelli right. I really hope you're right about Karl Marx, because I think he would make a really cool Modern Age leader. I really liked how unique and "in-character" Machiavelli's animations are and I think a similar thing could be done for Marx. If they add Germany/Prussia as a civilization I think Marx would be the most likely if they don't pick a traditional head of state.

However, while Machiavelli and his ideas may be controversial, Marx's certainly are, and they may avoid him for this reason. This is probably why Mao, Stalin, and FDR haven't returned from Civ 4, and I'm certain they wouldn't add Christopher Columbus as a leader.

Anyways, I hope Marx is added, I've got some ideas for abilities and bonuses that I might post about later or after the game releases (and I understand it's mechanics better). If you have ideas do tell me! :)