Some people just ignore the fact that superpowers just can not be threatened. This is just how reality works.
The only solution for real challenge, right now, after the point of owning 16 full provinces - is "Shogun 2". When all start to absolutely hate you, and declare war on you.
And this is not the best solution.
A.I. is really problematic only if he has like 5 times more armies and attacks from all directions.
1 vs 1 or, even. 2 vs 1 stacks, the A.I. factions have no chance.
This game can not be lost from the beginning when you are good enough.
C.A. need to improve campaign A.I. massively, to represent some middle game challenge. As I said. They are almost never make good ambushes. They are rarely attack with 2 or more stacks in one place, to destroy your city with only garrison. A.I. do not create army groups that swarm attack you. They do not concentrate forces, they spread them too much. And 10 stacks that killed 1 by 1 is weaker than 3 stacks that are fighting together.
It is not hard to fix this. It is can be easily coded.
What's more interesting, is intrigues. Like you have some non-aggression pact with one faction.
And some other faction knows that you are on war with the third one, and your forces are not near them. So, they declare war on you, and bribe other faction, that you had this non-aggression pact with, to simultaneously attack your undefended cities from 2 sides. This is how real politics and war make real challenge.
Right now, A.I. is a very bad and dumb politician. It is bad in planning to kill you when you are occupied somewhere.
They are too passive. They do not scheming.
And, right now, A.I. is very bad and dump strategist. Not ambushing and not striking with all forces in 1 place for sure wins. Spreading too much. No timing attacks.
Interesting that in real time games like "starcraft" and many other, A.I. knows what is "rush" when you are weak, or what is big attack with all forces at weak points, when he is strong.