I think Lemur's setting of ancient China set in the Ten Kingdoms period is the most likely, followed closely by a Napoloenic game. I would peg some sort of fantasy title as a distant third, but I don't think we'll see anything like that for a while yet.

A TW game set in the modern period (including both World Wars) would be a grave mistake. As King Kurt pointed out, technological development would be too rapid for a game like that to be enjoyable. Doing so would also move away from Total War's traditional (and successful) formula of huge armies clashing in melee combat -- if I want to play a game where men mostly die from shooting at each other, I can pick up any of the zillion WW2 titles that are out there.