PCs snap into a new reality. They are far far from home, on a rocky outcrop above a field of mushrooms and strange creatures. Things don't work the same way they did back in the real world. Some weapons do nothing, others are insanely powerful, and spells don't work at all. Creatures and their behavior there make no sense; they may meet other versions of themselves, or forests where you're underwater when you walk in the forest, things like that. You can't keep them there for too extended a length of time (it'll be too unbalanced and not-fun if all their usual gameplay mechanics are just gone for an extended series of adventures), but maybe there's a type of crossroads with a massive six-legged bear-like creature who says if they each give him one thing that's most dear to him, he can give them directions closer to their home. The directions take them to some odd astral place that's at least in the sourcebooks (feywild maybe) and their stuff starts working again, so they can start working on how they might be able to get back to the material plane. If they don't wanna work with bear guy then they're going to be stuck in feywild-squared with weird stuff happening to them for potentially quite a very long time.

Or

Fire engulfs the whole stronghold, all of a sudden. Either the BBEG keeps fighting, invulnerable to it, or else gets consumed by it while the PCs have to flee for their lives like the end of Metroid, and the BBEG presumably perishes in the ashes. If you want to make them work for their treasure you can hint to them that they should go back and sift through the ashes and they might find some neat stuff that wasn't flammable.

Or

Anything the BBEG does to the players starts impacting one of the players (chosen at random) instead of the BBEG. BBEG can hurt them but chooses not to, instead invites them to keep hitting BBEG but they're only hurting their other party members and BBEG is fine. Once they stop hitting BBEG, BBEG gets really angry and starts chasing them out, telling them to get out get out get out. The next day they discover that they are still getting injured when stuff happens to BBEG, and the new quest is to either figure out how to undo the wish (nigh impossible) or else undo the insanity so that the BBEG will no longer have awful self-destructive things happening to BBEG which will then come back at the party.

Hope these ideas are somewhat useful

To be fair, a wish spell is about as close as mortals get to deific power, and IMHO should be treated as such to the fullest extent. Complete with reality-bending, self-contained "logic" that does not ask for permission and of course the high likelihood of extraplanar intervention - commensurate with the spell's resulting effects (and their consequences)...

If this BBEG is as unhinged as you say, as detached from the collective hallucination that is "real life", then not only would he throw the gates wide open to choose whatever fate he desired at that exact moment, but such reckless disregard for the foundational fabric of existence is certain to piss someone off who has the power to stamp out his spark (or revert it to its first incarnation, et al).

Personally, were I in your shoes, I'd put the Fear into the players by choosing something so ridiculous, so surreal, that the gods wouldn't feel like getting in the way and simply chalk it up to a mortal soul fizzling out in a psychotic firework. Something that would haunt them for the rest of their days, and maybe cause their story to be doubted or even disbelieved — no matter how they pleaded their case. The heroes that couldn't accept failure wove a tall tale to save face, but in fact brought an unknowable, insatiable curse on the region...

What if this BBEG decided to transmogrify himself into the original source in that world for all microscopic parasitic life? A germ? Or, even more esoteric: a pervasive superstition that defies logical dissolution. Perhaps, instead, he merely wants to forever be in complete control of a specific shade of mauve; he is mauve and every speck of it is an extension of his consciousness.

The outside world might view him as insane, but don't forget: to himself, he's a singular genius and makes all the sense in the world!

"I wish this dog pile were actually a pile of dogs."

I wish, you could see it my way.

Bbeg is now besieged underdog.

Important clarification: is this the end of the campaign?