I have a few different ideas, but if this gains traction I would want to hear other ideas. Also, I can't evaluate the implementation for technical feasibility/difficulty. If I ignore those things and just 'dream' about how I would implement it then here is just one idea:

There is a latest-game tech that allows you to build a star-base improvement. This improvement converts the star of that system into a wormhole, destroying the system in the process. So you build one and, WHOOPS, you just released the Dreadlords. They begin sending their endless ships and superior technology through the wormhole invading real space until your civilization is destroyed. A turn counter prominently displays how long you last for.

This is the core idea, but there are a lot of things you can add to make it cooler:

- Maybe after destroying your first Dreadlord ship your scientists unlock a new "Age of Dread" on the technology tree. This Age has some near-game-breaking tech that is ridiculously expensive. The final 'nodes' in the age allow you to continually research them for cumulative bonuses to near-every area of your society. You choose what to prioritize. The Dreadlords are strong against you because they start way up in this tree and have 10x your research points per turn. One Dreadlord ship should be able to defeat a fleet of obsolete/non-upgraded ships single handedly.

- Maybe, instead of it being a late game tech you can instead build the wormhole at any time in the game. This would be a cool option if you knew you were losing, but you would take pleasure in knowing your AI opponents will die with you. The final rounds in your losing game are more interesting as you navigate a diplomatic scene where enemies have become allies against a bigger threat. This way even losing your games is fun as you try to outlast your opponents in the new political landscape.

- So if you build the wormhole prior to wiping out your enemies then the Dreadlords appear and start their killing spree. However, if you wait until all of your enemies are conquered then you get the bonus of being able to set up your worlds & fleets as you'd like, and complete all research before opening the wormhole. Ideally there would be no 'turns', just unlimited money, movement, research. This would account for you being able to open the wormhole whenever you want, while saving you from the hassle of clicking 'Turn' 300 times before opening the wormhole. For those of us that like micro-managing our empires to make them as good as they can be this would be a fun excercise. We would get it all perfectly set up before unleashing the Dreadlords.

- The time/power the Dreadlords scale at could be selected by the player like the game speed can be. Maybe they come out and destroy everything in their path. But just as fun sometimes would be this slow, steady, unending build up over time.

- Maybe there is another option in the game setup: On turn x the Dreadlords return, regardless of whether you built the wormhole. All civilizations know it's coming. The goal is to last as long as you can.

Another completely different idea is this: After you win/lose the game your map size is upgraded to the next size up with the here-to known universe in the middle, and the Dreadlords own the space around you. You are encircled, on all sides, by your new enemy.

I had a bunch of things listed, but then the internet ate my post, and I can't remember them now. So I will update this as I remember if the thread/idea gains traction. But this is just scratching the surface, and I'm sure others could come up with both better core ideas, as well as better 'make it cooler' ideas. The key is to make the game playable and fun after the victory/loss. That way the need to be 'on-top-of-things' doesn't diminish as you get closer to the end - you are always preparing for the next enemy.