Threat from another galaxy

After pacifying the entire galaxy, I've been thinking about how to make post endgame more interesting.

To the Star Wars fans out there, remember the Yuzhan Vong?

Well I was thinking, how about, once all the enemy factions have been conquered and peace has been brought to the galaxy, a new threat in the form of an unknown alien race (similar to the Yuzhan Vong) comes from another galaxy to destroy ours. 

It will be one of the most powerful enemies you have encountered yet and it will take almost all of your resources to defeat it.

This event takes place in the form of a quest/coop quest that can only activate once your have pacified the galaxy. 

In order to complete it, the allied players would have to band together to defeat this formidable opponent.