I think MTW2 was a logical decision. Most people don't know anything about feudal Japan, but they can relate to shiny European armored knights on horseback. In that respect, I'm kinda surprised they did RTW, since these days there isn't much popular knowledge of ancient Rome outside Cecil B. DeMille movies.

MTW2 may also be a springboard for Napoleon:Total War as the next major game, which would also make sense in terms of popularity, but I hope they don't go there. I prefer fighting with ancient, non-gunpowder armies. So I hope they do something like China/3 Kingdoms as the next one. Heck, I'd settle for Stone Age:Total War. Anything that doesn't have firearms as the main combat style.