I like this idea, though I have to say that the depiction of the gods should be a little more subtle than in Age of Mythology... after all, this is a historical game, not a mythological one. The gods should work like a special technology. As for keeping favor, you should probably just loose a trickle of resources to represent the sacrifices. Were each nation to have three gods (which seems like a good number) for the Romans I would have: Jupiter, Mars, Quirinus or Juno, Jupiter, Minerva (the old and new Capitoline trinities, respectively) For the Ptolemies, certainly Ammon, Isis, and Serapis; and for the Carthaginians Astarte, Ba'al, and Dido/Elissa, or perhaps Tanit.