Please give us an Endgame Challenge

I play often coop with my friends against the AI. The start can be hard, if you start right next to a strong AI you may lose, before any of your friends can come to the rescue.

But if you survive the start and early midgame you've almost always won. Then winning is only a formality, and not actually a challenge anymore. If you make the AI stronger you almost always die in the early game.

Nevertheless it's still balanced enough to be fun till the late midgame before it becomes boring.

As a solution, please add a feature that has been proven to be working in other games:
Add a super strong enemy in the endgame, as an optional feature of course. Which is so insanely strong, that you must beat it with special tactics, using all your special abilities and that of your allies.

Yes, I mean: Add an End-Boss! Like an Alien-Race, coming from outside the galaxy with insane powers, for a really challenging boss-fight. Oh: And of course it should be possible to configure the time when it comes in, everybody has their own playing speed, and a different time when it's his or her endgame.