OP has a point, game gets boring after a certain point once the outcome becomes clear. One may attempt to find a solution with some creativity and imagination, a couple of ideas:
Open objective: Final battle
- A final battle with the Enchantress is required to win, the battle is accessible anytime but the power of the Enchantress is very high early game and continues to increase as time goes, it also scales with the number of remaining AI players or artificats/Gaurdians/Dragons... etc, a sort of doom's day countdown but with no timer, you can always challenge her if you think you are strong enough.
Scenario: The World unites
- Human player alongside AI players must survive a powerful 3rd faction preferably a unique unplayable race [Undead/Zombies] with great advantages, 3rd faction goes full offense after random XXX turns, Human player needs to balance between grabbing the limited resources for himself and at the same time not over-weaken other AI players, as each defeated AI player will considerably scale up the power of the Zom.. 3rd faction
Scenario: God's challenge
- The king/Demon lord/Dragon queen/God/Satan... announced a tournament between all factions. Human player alongside other AI players must explore the world seeking to enlist the aid of Heroes, dragons, witches, demons, golems...etc by conquering dungeons, solving puzzles, raiding cities, doing quests. Player will still need to build a city and an army for aid, as well as try to hinder/Kill allies of other AI players.
After a random [Unknown] number of turns, everyone is called and you are required to pick a number of allies to join you in the battle for survival, player will be able to see AI vs AI tactical battles of other players as well as his own. An interesting twist would be that the player cant control his units in the tournament, so recruiting smarter units maybe as important as adding yet another huge damage dealer.
Scenario: Odin's call
You are required to descend to earth and build the strongest army possible, after XXX turns you (and your army) are transferred to heaven (just another map) to fight alongside Odin's army against a powerful army. In the new map you can no longer build new cities/units and must utilize the army you built/Allies recruited to win.
Well, that's enough ideas for now, perhaps modding can remedy the problem.