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!