For the future I would love to see an American Civil War game though I know it doesn't really fit into the "Total War" setting. I also quite like the idea of a WW1 game, though the issues with long range weapons ( plus air) would make it a very ambitious project.
Trench warfare and continuous front was such a big part of WWI, I'm not sure how TW (which tends to favor open field battles, small armies, and lots of manuevering around forts) would handle it. Plus, I get the idea Empire/Napoleon was about as modern as the TW folks wanted to go.
Actually, in March the developers at CA are releasing a stand alone expansion for Shogun called: "Fall of the Samurai" which is initially set during the Boshin war (1866 to 1868).
This was the final overthrow of the Tokugawa Shogunate by forces loyal to the Emperor. By this time all the European powers were selling weapons to both sides, so in addition to traditional weapons, the combatants will now breach loading artillery, Gatling guns, Snyder rifles, and Ironclad based naval battles--plus some foreign recruitable troops like British, French, and American marines.
The developers have already stated that they will not make two game sequels in a row, so Rome II will not be the next major release. From the period of the new Shogun II expansion, many are guessing the next game is going to be at least in the 19th century--maybe a Victorian Total war that would include the ACW and various colonial wars of the late 1800's. Of course it's all speculation at this point.