The only somewhat sensible way I can see that happening is if they lower the quality of the troops and focus more on quantity and special abilities like "fear" or something along those lines.
Fanatic traits also! Nearly unbreakable units.
But I really hope they make up some new traits, or simply invent some new mechanics. They have been milking the same meta-game for years.
Also siege mechanics blog http://wiki.totalwar.com/w/Sieges_and_Siege_AI
I think its nice ideas that they "focus" the sieges to a few walls, and create bigger walls, streets, while they are fewer; more intense fight, and still hard sieges.
But gotta call BS on the AI being able to manage reserves, have multiple strategies and split his forces in a coherent manner. Or they finally did some AI work after all these dreary years.
If they really do that and we dont see any more AI bumrushing the gates, then its gonna be something to behold.
Still gotta remember that each time they release a new Total War, they make up some nice dev blog about their AI being ultra better than last time... Dont read everything like its true.