Actually, I've heard that the next Total War will be a rework of MTW for the RTW engine. Just rumors and innuendos, of course.
As for the Rome expansion, I dunno. Maybe be a prequel like MC said, Alexander or earlier. Of course, maybe it will cover the fall of the Roman empire; Hunnic invasions and the Constantinople - Rome split.
Whatever CA (or in this case Activision) decides, I hope their next releases give modders an easier time.