I really want a sequel with more medieval leanings.
Something where you could have a European civ of Christian/ Jewish legendary stuff (Dragons, Camelot knights, maybe even some Irish faerie folk things), Indo- Oriental culture (Hindu/ Budhist/ Sikh folklore stuff), and maybe Aztech or Middle-Eastern (Arabian knights, or Mezzo-American things).
I know this is highly unlikely, as drawing on more familiar cultures tends to lead to all kinds of odd places in this day and age, but I think it would be cool to have a knight charge with dragons going up against a force of Naga warriors.
Another possibility I’ve wondered about; letting the Age-up choice explicitly split down a “light and dark” path. A little more than what the original AOM did, so that you could either guide your civ down a more “pious” road, or actively make them follow darker and destructive forces. To be fair, this would possibly be drifting a bit further from the Age formula, but I do wonder if it couldn’t happen.