I'll second the others who've said that real-world religions are probably not going to make it into the game. In general, I feel like real-world cultures shouldn't be directly reproduced, but rather that The Sims team create something that is reminiscent of them and allows that kind of world to be played, but also feels like the Sims version of that world (i.e., not a copy of our world, but something a little off-kilter yet similar).
That said, I would very much like to see another ancient civilization incorporated into the game that we could explore and learn about. And the Greek / Roman / Mediterranean ancient cultures make a lot of sense for this. There are even some small things already in the game from other packs that could be referenced in this new pack. For example, I wonder if we would ever find an ancient building dedicated to Gluteus Maximus (referenced in two objects in the Romantic Garden Stuff Pack), who apparently was the equivalent of some kind of Roman general in this world. This is the sort of small, absurd detail that makes The Sims into what it is, so if we do go the era of antiquity in a future pack, I'd love to see it done in this sort of humorous way.