If there is a new type, and it doesn't turn out to be fairy or another name for fairy, we should look at the roots of X and Y. As we all know, X and Y are chromosomes. The game is rumored to focus of pokemon DNA and such.
Perhaps the new type could relate to this? I've heard suggestions of "DNA" or "egg" type, but those both just don't sound right... (The egg type suggestion was made after the reveal of Sylveon in a pink egg symbol). But maybe we are looking in the opposite direction, maybe it has more to do with science than it does with mythology.
That being said, I like the idea of a fairy type.