My guess is that they'll go for Rome 2: Total War. It was an extremely popular title and probably the most popular historical time period they've ever done. I also believe that if they do make a Rome 2: Total War, it will be much more historically accurate than the first Rome. Not up to EB standards, to be sure, but definitely better than the original Rome; because that was the low point for the series in terms of historical accuracy and they actually do seem to be making much more of an effort towards historical accuracy in Napoleon.

As for what I'd most like to see, it wouldn't be Rome 2. EB2 will already fill that niche fine for me. I would love to see a Dark Age: Total War, like Hannibal Khan said. They've never really done that time period, other than Viking Invasion, and that was only of Scandinavia and the British Isles. A full Europe/Middle East campaign in this period could be very interesting.