Actually, I think that having Aztecs in the game would be rather interesting. It would be nice to choose between Quetzalcoatl (One of the sun gods), Huitzilopoctli (god of war and protector of the sun), and Tezcatlipoca (arch-nemesis of Quetzalcoatl and god of darkness). I just wonder how they'll gather favor, though. Praying at a temple is one thing, but human sacrifices would be rather,...well...neat.
As far as myth units goes, I had an idea for an almost uber-myth unit in an age later than the mythic. You know, something that could almost be a one unit army, but could be taken down by about one-two heroes. The only twist is that they are expensive and you can only have one at a time. For the Greeks it could be Cerberus, for the Norse it could be Jormungand, the Midgard Serpent. I just have to think of one for the Egyptians. Maybe it could be a different myth character for each god. Hades gets Cerberus, Poseidon gets the Kraken (like Clash of the Titans), and Zeus gets a bigger Cyclops. I don't really feel that it would be unbalanced because, even though they are strong, heroes can still bring them down. To make things even more interesting, it could be something to the effect of "heroes get bonus damage against a myth unit, but a particular hero gets a bigger bonus". This basically means that Heracles has a bonus against Cerberus, Jason against the Kraken, and Oddyseus against the Cyclops (I know Polyphemus is already in the game, but one can dream.)
The only problem is, I don't know how that bonus would work out when playing Greek vs. Egyptian or Norse. It onlly really works with the 'named' heroes. Well, enough of my rambling for now, I just had to put that idea out there.