Not sure if she counts, but my favorite Xeno artist will always be Soraya Saga. Even if there's very little Xeno between projects, there's always some that crops up after a while. I know she's with the Monolith team, but there seems to be a very, very, very limited section of hers displayed within the official production works. The stuff that's the most visible is the stuff she just posts up on her twitter, pixiv and DA accounts, which is on the same channels most fanart is displayed. Her aesthetic is unmatched by anyone else I can think of. I really love it when she posts anything at all, too. Her art is just so good, asdfdasfs.
Another favorite fanartist of mine is LilyKane, who currently goes by LMJWorks on deviantart. I don't see her anywhere else, but I really love all of her work.
Dakkun also watches my art blog on Tumblr. *A* I have to stop and re-contain myself every time he likes something that I post (which seems to happen a lot...!!!). He does seem to have acquired a taste for Dragon Age, Mass Effect and Lord of the Rings lately, but as with Soraya, I enjoy all of his art.
I've seen Salyut on tumblr as well! I'm actually pretty glad to see Japanese fanartists on tumblr.
Also, a second motion on Stitched's shoutout to friends. :3 Yall rock and are super-awesome!!
There were one or two Japanese artists I'd found who drew a high volume of Ormus-centric stuff back in the day, but I never got their names and I haven't looked for them in a while. There was an artist who liked Shion and Cherenkov together and her(I think she was a girl? If not, then his) artwork seemed heavily influenced by the DS game. I've lost the URL to the site, though. -_-;