I'd suspect a new sea battle system, and the same crappy strategic AI and ahistorical all against all conquer the world campaign as in earlier total war. The strategic AI has suffered from the same limitations throughout the series. It seems like CA wont learn that it doesnt add difficulty to have weak neighbours attack you with low level units turn after turn...
Ain't that the truth. The strategic AI does need a major overhaul. Some things I'd like to see changed:
(1) Intelligent alliance-building. Have countries band up against each other, instead of just changing, proposing, and breaking alliances pretty much completely at random. In my last game as the HRE I had exactly three contacts with Byzantium -- once they made a naval landing and attacked Bologna, then they sent a diplomat to propose trade rights, and then another one to propose an alliance. I had no common borders with them, and only one common enemy. Hello?
(2) Intelligent decisions on war and peace. As you say, it makes no sense for an enemy reduced to one or two cities to keep sending poor stacks against a neighboring empire; they should sue for peace, then try to build an alliance big enough to take on the bigger enemy. Similarly, it makes no sense for the AI not to accept a ceasefire and trade rights if there's no invasion imminent.
(3) Simple decisions on strategic posture. Hell, just have three different types of postures: take and hold, raid and pillage, defend. For take and hold, build up a strong stack or three, send them to take a limited number of neighboring settlements, then sue for peace. For raid and pillage, send the stacks on deep raids of enemy territory, sacking and then abandoning every city they take. For defense, keep them on home territory but aggressively attack anyone crossing the border. Shouldn't be too hard.
It's still an enjoyable and light hearted game though, but one has to remember that the tactical battles are the heart of the game, and the campaign sort of an afterthought....
That it is. I think of the campaign as a reasonably interesting excuse for the battles.