https://tvtropes.org/pmwiki/pmwiki.php/Trivia/Firefly
"Joss Whedon gave an interview around the time
Serenity came out, where he stated that, had the show lasted multiple seasons, he had an idea
to include a cameo by Spike sometime around Season 6, retroactively incorporating Firefly into the
Buffyverse. The scene would involved Mal encountering Spike in a bar on one of the border planets, where the British Vampire would resignedly tell him "Nothing ever changes"."
This would have been both fun and sad.
But even the fact that Whedon thought about it, makes the Slayer Theory in a more real position than before.
P.S. The grammatical errors you can see between the "" aren't done by me, I've just copy pasted it.
anyways from what I see, it's a reverse situation of EMH - W&X-MEN - SSM.
The difference is that they were meant to be separate, but then they were made together, here they were meant to be canon, but they never had the opportunity to show it and never said otherwise. So I mean, I can see a Buffy/Angel/Firefly/Serenity universe.
You could prolbably add Dollhouse too, but there wasn't a single confirmation about that ever.