I wonder if Mashima is trolling us with that little girl's magic, since it wasn't explicitly stated that she has the same kind of magic as Zeref (only that it comes from the coliseum), plus that she is strong and that Mavis is interested/recognized her magic.
Well, it could be interesting to see the Sky magic vs the Underworld magic, since it would fit the opposite element match up.
BTW. Mavis surname - Vermilion ( I think :P), in some other works it is associated with a powerful demon capable of stealing & burning life. Sounds a little similar to what Zeref can as well, perhaps they are more closely related than we think.