First of all I just want to say thank you all for having this community, I just found it and it's right up my fantasy alley. I've read maybe a dozen stories in the past 24h!
Anyways, I've been having a particularly twisted fantasy that I'd like to share to possibly inspire someone here to write out the scene details...
The basic premise is that the man would somehow (maybe with a genie or magic object) get to make a wish, and that it would be in line with "wishing to know how his ex feels". Rather than be a pure body-swap or possession with agency though, the wish would make him become trapped in her mind, behind her eyes, feeling and seeing everything she did with her having no idea of him being a passenger.
The kinky part could come with her (being freshly single) wanting to be a total slut and the ex being subject to her escapades.
...alternatively... it could also take a turn towards her visiting her ex to find him having apparently vanished, and then finding the wishing object at his place and haphazardly turning herself (with the ex still trapped) into a bimbo or worse.
This is partially inspired by a story read elsewhere years ago about a couple on the beach finding a genie's lamp. The boyfriend quickly wished to bimbofy his girlfriend but before his girlfriend was completely helpless she grabbed the lamp and wished for them to switch places, causing him to deal with becoming the sex doll version of her. If that rings a bell with anyone i'd love to read that one again too!
Cheers
; )