Being an ancient theme fan, I think this is a great poll! Here are my thoughts:
Asian: I know other members would love this theme and I think it could be quite spectacular, but I personally have no interest in it. It also wouldn't really work with any of Playmobil's current historical themes.
Greek: I would really like this theme and I think it would be a nice complement to the Roman and Egyptian themes. If Playmobil produced some Greek buildings, it would satisfy Roman fans too. Just imagine the Parthenon, the Trojan Horse, Greek chariots, and a heavy phalanx warrior set!
Roman (expansion): The Roman theme is my favorite theme, partly because I'm fascinated with Rome and partly because I think it's a fantastic looking theme in general. Playmobil did a great job with their initial run when it came to the military and gladiators, but they completely neglected Roman structures, except for the arena. A Roman forum, villa, and temple would be welcome additions to the theme, and I think they could expand the military side to include some green auxiliary soldiers and some black or light blue Praetorians. Playmobil could even further expand the Roman theme to include a Celtic chariot, hillfort, and round hut (which could work perfecdtly with the new African theme too).
Egyptian (expansion): This is among my favorite themes and I think Playmobil did a tremendous job with everything. I can't imagine them doing much more to expand this theme.
Persian: Although I don't think many will choose this, I would certainly like to see a few Persian sets, as they would complement both the Roman and Egyptian themes nicely, not to mention the proposed Greek theme. I'm not sure, however, that I could see an entire theme of Persians.
Maya: This theme, like the Asian theme, could be quite spectacular with a temple and some great klickies. It would also work well with the new Spanish conquistadors in the pirate theme.
So my order of preference would be:
Greek
Roman (expansion)
Maya
Persian
Asian
Egyptian (expansion)