I've been having fun recently writing a crossover between two of my worlds. Lexi, a P.I. from the regular world, investigates Laurel, a witch from a regular-world-plus-hidden-magic. A major part of the story is Lexi freaking out as she learns about the things lurking in the shadows.
I assumed from the start that I'd need to declare it as non-canon upfront. As much fun as it is to throw Lexi down the rabbit hole for one story, it can't be real to her without undercutting all of her past stories and locking me down on future ones.
The thing is: everything that happens from Laurel's world's perspective is completely reasonable. And a lot of it follows quite naturally as what should happen to her after her last story. When I go to write her next story, I'll be inclined to think of those things as having happened.
Is it fair to say a story is canon in Laurel's world but not Lexi's? Is it fair for a writer to declare some of their works as non-canon at all?
I don't consider this an actual problem - just a philosophical curiosity. I'm interested in what you folks think.