A potential problem with the endgame scenarios is that you change the already established win conditions in a way that could feel unfair to the player if they have a "certain" victory snatched away from them.
You could instead introduce extra endgame objectives. You still win regardless but completing them for "bonus points" gives you ...
One thing you could do is give the AI an "all or nothing" response to losing. If he thinks he's going to lose, he could do something crazy like, spend all his money on the lottery. Gather all his forces and attack your fortress. Invest all his money into research for a new weapon. Hire barbarians to attack you at the risk of getting attacked ...
There are lots of ways you could accomplish this, I'll just give you one to get started.
Attach this script to a gameobject, and give it another gameobject as target to follow. Uncomment line 29 if you're using a top-down view.
public class AssassinMove : MonoBehaviour
public Transform target;
[Range(0, 10)] public float speed;