Because the Eevee-evos come in a set (fire,water,lightning)(psychic,dark), I would hope to see dragon & flying evos. Think it would be quite original; Eevee w/ wings......
(I'm actually surprised they didn't make Eevee-fighting with Espeon and Umbreon......The three make a perfect set IMO)
I voted for a ghost-type, I really wonder what the ghost-type eeveelution could look like.