Now I haven't managed to make it through all the archives, but has there ever been any discussions on Clamp's X/1999 or Tokyo Babylon?
If not, I might as well start the first one.
I ship the very canonical SeishirouxSubaru of Tokyo Babylon and X, a gay pairing and one of the most psychologically disturbed of Clamp as a whole. I also ship SubaruxFuuma, SubaruxHokuto, SubaruxKakyou and SubaruxSetsuka, showing that I'm not adverse to at least playing with other ideas when SeishirouxSubaru isn't quite what I need today. And I'm usually open to most other Subaru ships because they run a fairly good chance of having him in all of his angsty glory.
What I'm not so fond of, is SubaruxKamui. Maybe I'm looking in all the wrong places, but so far what I've seen is a lot of stories that marginalise or erase all of Seishirou and Fuuma's effects on the two of them, and claim that it's the only "fluffy pairing that Subaru and Kamui can have."
Unless you take the time to develop a relationship with all of the issues that those two have over a reasonably long period of time, my answer will always be, "Um, no."
If you want a fluffy pairing for Subaru, the only character who has the knowledge and gentle interactions with him minus the psychological issues is his twin sister. If you want a fluffy pairing for Kamui, he has a much broader range; he can be reasonably paired up with Kotori, Keiichi, Sorata, Yuzuriha and Fuuma before the events of X/1999 really come into full effect.
Write it if you like, I'm sure I'd like a well-done fic about them that acknowledges their issues, acknowledges that they will never fully heal from the scars that they bear, but I will not look at it as "Subaru's canonically fluffy pairing."
Because it really isn't.