GREAT IDEA! I have a campaign which is currently ending which this idea will work PERFECTLY for. BRILLIANT!
Well this idea occurred to me at the last moment. Basically the PC's had to heal a rift in the fabric of the Multiverse - which meant that each of their Multiversal personas (from all the other game systems we had played through) had to seal up the rifts on their own worlds by stepping into the 'null space' portals.
Null Space is simply 'A realm of total nothingness'.
As they tried to seal the rifts the villain of the piece ‘Endgame’ sent adversaries from the other realms to stop them. We had Deep Ones and Nightgaunts fighting Elven heroes and MI6 agents struggling against orcs and goblins - general reality chaos across he multiverse ensued.
Oh, and (I liked this the best) it was made clear to all the incarnations of the heroes that the reason they had all become adventurers was to one day stop this event from destroying reality.
I deliberately left the concept of ‘The Null’ open – as the heroes stepped into the void of nothingness and sacrificed themselves everyone asked ‘are they dead?’
I simply closed my notes and screen and smiled. They may return one day...or not.
Who knows? For no one has returned from The Null...
Then we had a few cold beers - and the next evening was a hell of a party!
BTW - Endgame was a cosmic amalgamation of each of their single greatest major villains they had fought across the realms (although I hit on this idea towards the end!)