This question is only to be answered correctly by Game Freak however it is most likely that a new plate will be introduced seeing that Game Freak keeps adding new features to X and Y
But we cannot conclude this however as they have not mentioned anything about it. Putting our hopes up for it is also not recommended as this generation seems to be following a new pattern for example the Mega Evolutions. Who would have thought that Eclair Mewtwo was actually Mewtwo's Mega Evolution? No one. Why is this, because everyone still had the pattern of legendaries gaining new forms in their heads but Game Freak thought otherwise and decided to do something new
Also to add, the Fairy Types seems to be a very special type and since many Pokemon that were formally normal types or neutral types have become fairy types, it is very possible that Arceus's changes type do (Note, this is just my thought, Don not take this into account)
So I recommend that we should stop trying to figure out Game Freak's infinite mind, and just wait and see what they have to offer us.
However, I do think that they would make a fairy plate for Arceus :)
