From previous experience, I've found that DGM and Blue Exorcist mash up pretty easily (the demons in Blue Exorcist are technically from another plane of existence, but possess humans and other animals/objects to manifest on Earth. So it's pretty easy to have Akuma be a type of demon that specifically possesses corpses). Additionally, several of mine can fit into other canons pretty easily, mainly because their canons are pretty normal (Satoshi, Mafuyu, and Nanaki are all like this). Ivy is from the right time period for D Gray Man, and her world is steampunk/fantasy/horror monsters everywhere, so it's not too hard to fit her in, or to fit D Gray Man characters into her world.

Also I have to admit, it would be pretty funny to play Satoshi off of Heine, if only because Satoshi looks androgynous-enough to be mistaken for a girl at first glance. Likewise, Mafuyu is a girl, but cross-dresses in canon and makes for a very convincing boy.