Maybe it's just that the medieval setting fits in so well with the Total War formula, but good god a 3rd Medieval would be amazing. Thoughts? Besides the obvious improvements, I can think of a number of things that would make a second sequel really shine. Here's a bullet pointed list!
-Siege towers and ladders that aren't just death traps that conveniently funnel your troops onto the business end of a spear.
-The addition of the one shot kill mechanic from later games in the franchise; i.e. one musket shot in Empire will take out one enemy troop(usually). This would probably be better as sort of a side mode kinda thing rather than a mandatory addition, otherwise entire squads of units would be wiped out in about 10 seconds. It works in Empire because the guns take ages to reload and shoot.
-Cavalry charges actually charge THROUGH groups of units, rather than kill a few and then get stuck and consequently butchered.
-Actual navy battles. Wouldn't be as in-depth as Empire or Napoleon due to the era, but it would be nice.