I think, or at least hope, that they'll make a game covering 1500 to 1918 AD, from the first muskets and colonization to the Napoleonic period, the revolutions and later colonization and then the beginning of trench warfare. It would be something that the gaming industry hasn't created yet. I certainly hope they add 3d player-controlled naval battles too - in the beginning of the era with galleys and boardings along with carracks and similar with early cannons. The battle of Lepanto would nice to play (the battle of Sluys wouldn't be bad either, although it was much earlier, back in approximately 1337 or something like that)...

Or alternatively they could make Medieval: Total War 2, with naval battles and better diplomacy and AI than in R:TW, and with more factions.