After ten months, I was pleasantly surprised by the return of Mightygodking’s I Should Write Dr. Strange, and with a plot hook where you could substitute Strange with Who again.
It’s an example of the classic “happy future or alternate world as a trap” - or is it?!
But it also raises the possibility of a full-time crusader like a Chosen One retiring. The Dark Knight Returns has been ruled out as the true final chapter of Batman’s story several times over. Marvel Comics have a series of The End stories for various characters, but I don’t think any of them are counted as canon. Buffy has a prophesied Final Battle which she still hasn’t gotten to (as seen in the future spinoff Fray) but even it wasn’t as final as all that.
“Always in motion is the future.” (or “always emotion...” One of those... maybe both...)
A nightmare future gives the PCs an obvious fight on the way, and maybe the knowledge they need to win it, but a happy ending will probably lead to a mystery of how it could possibly happen. There’s a cheery thought...
So there’s always room in a game with enough Weird Level for a prophecy, portent or visiting time traveller to present a flashforward to an ending the characters would or wouldn’t want and then snap back to the present and see how they react, which might well change the future right then and there. Or a definitely non-canon one might be pure speculation, to be played as an isolated one-shot and only known by the players, not the characters.
Setting this up could be guesswork, or you could discuss it with the players, either in vague terms like “what does your character plan for the future... and what do you plan for them?” or in more detail if they’re going to play their PCs’ future selves while a few of the PCs play the current selves seeing the future.
It’s easiest to build a future where the turning point is known - for example, if the PCs fail to stop this season’s Big Bad, the future will probably be rather grim.
Alternatively, a far future avoids the “my character wouldn’t do that!” factor. If it’s seven thousand years later, the history people know of the heroes could easily be inaccurate.
Consider why the portent happens. Is it a warning, a temptation, an attack, an accident? These would all lead to a different version of events being shown.
And of course, even the tempting happy futures
allow you to kill off lots of characters in the interim, suggest events
that probably will happen in the near future, introduce NPCs (and
even PCs) who the current PCs have yet to meet normally, and of course
give future characters badass scars...