The next Total War game will be announced at Rezzed over the next few weeks, although we may already know exactly what we're in for. In a teaser for the next issue of PC PowerPlay, a picture of Rome: Total War is shown with a giant "II" emblazoned across the battlefield and, while this technically doesn't confirm anything, there isn't a lot left to the imagination and I find it hard to believe they're hinting at anything but a sequel to 2004's Rome: Total War.
Rome: Total War was a fan favourite and is long overdue another outing. If you're not sure about the series, or at least about the Rome aspect of it, a Steam sale is supposed to be on sometime soon (perhaps even this week) and it's always a game that's reduced. I think I picked it up, along with its expansion, for around the £3 mark a couple of years ago.
Expect an official Rome: Total War II announcement on either the 5th or the 6th July.