So I'm GMing a supers game for a few friends and all is

>>50053508
Steal from the justice league cartoon.

>webs in the fabrics of space are starting to unravel for some reason
>could be a cosmic horror just taking a stroll through the neighbor hood, some powerful wizard doing it for shits and giggles, or an alternate reality where good guys,are,the bad,guys and vicer versa
> this is causing all earths of every reality to occupy the same space in time

>world ending

It would probably be the only reason why villans and heros would team up

Take a note from a grimdark web novel called Worm. There are creatures of massive power and destructiveness that regularly attack cities and even more regularly kill at least 25% of all superhumans that show up. There's a whole list of unwritten rules and truces that allow everyone, hero and villian, to set aside their beliefs to defend humanity in these fights.
>The signal?
Air Raid sirens.
>Lethality?
If you get hit by an endbringer and don't have a high durability power you're dead. D.E.A.D. Roll a d4 for every NPC that shows up and if that NPC rolls a 1 they die. That's literally how the author did it even risking the MC.

>>50053508

Other reccomendations in this thread were good but if you're still looking, there's always the whole "perfect being gone wild" thing. Maybe one of the villains or even one of the heroes tries to build an ultimate lifeform that will slay enemies or keep the world safe without the need of heroes. However, it gains its own free will and comes to the logical conclusion that if there are no heroes or villains left alive, the world wouldn't constantly have to deal with the fallout of their schemes and battles.

Notable media that does this or something similar: Age of Ultron, JoJo part 2, 2001 A Space Odyssey, The Day the Earth Stood Still (the newer one is shit). In fact you may wanna ask /co/ for some more examples, there's a few that are eluding me at the moment which I'm sure could help you with your writing.

>>50053588
>Batman and joker in one of their fights while driving 200mph on the highway
>joker gets thrown into a river, gets out, has a fist fight with batman, and almost dies.
>As batman winds up for the knockout punch an endbringer siren rings out.
>He pulls joker up and then drives both of them to city hall to congregate along with every other hero, villian, and independent cape in the state to prepare against whatever monster shows up this time.
I can see it.

>>50054119

What would those two even do against an Endbringer?

>>50054266
Batman brings out his super secret endbringer killer device and the joker runs semantics and strategy with his claws in the back pocket of nearly every supervillian in the city.

>>50054544
>semantics

Perhaps you meant "logistics?"

>>50054731
Yes.