The problem of a boring lategame where you've become essentially unstoppable, but still need to go through the motions in order to win, is a problem in nearly all 4X games, FE included. While the problem may seem to lie in the win conditions being too high, in my view, the problem lies more with the AI failing to respond as your empire goes from "powerful" to "unstoppable juggernaut". Continuing their regular strategy is what allowed you to get strong in the first place (and would therefore make the rest of the game nothing but mop up), so they obviously need to change things up.
This could go in several directions, and should be based on both the AI priorities and their relationship to the player. If they've been friendly or neutral, they could seek out an alliance or vassalage, otherwise, they should choose between a straight-up attack (if they're near-equal power) or a focused, all-in attempt to hit hard where you're weak (fast units to attack lightly-defended outlying cities, AoE vs clustered trained units, buffs+debuffs+lots of high damage units vs a champion heavy army, etc). Diplomatically inclined enemies could try to bring in others, the less diplomatic would go it alone. And, importantly, if their last ditch attack fails, they surrender right there - ending things with a bang rather than a whimper.
Thoughts? Anything to add?
