Are there anymore recommends like https://www.fanfiction.net/s/10225608/1/A-Demon-Among-Devils
I just read the whole thing and really need more like it. Seriously great find on that. I'm usually apprehensive of fanfictions and crossovers in general since they are non canon but that was just too good to deny.
The only other thing that comes to my mind right now is Ryuugi's The Game We play, a great crossover between RWBY and The Gamer. While, it's on fanfiction is not update regularly like it is on spacebattles: http://forums.spacebattles.com/threads/rwby-the-gamer-the-games-we-play-disk-four.311394/
Edit:
Just remembered another interesting crossover between Fate/Zero and One Piece: https://www.fanfiction.net/s/10923264/1/Invictus
Another interesting fanfiction, a great take on Mass Effect AU: https://www.fanfiction.net/s/10804452/1/Mass-Effect-Synthesis
A great take between SG1 and dresden files: https://www.fanfiction.net/s/8005922/1/God-s-Eye
While, this hasn't been update in a year is a great crack fic: https://www.fanfiction.net/s/10093207/1/No-Need-for-Halkeginian-Logic
Kingdom Hearts SI(Self insert) fic: http://forums.spacebattles.com/threads/heartless-kh-si.276762/
All expect Invictus can be found on spacebattles: http://forums.spacebattles.com/forums/creative-writing.18/