The only logic eeveelution would be a dragon-type. Come think of it, Fire, Water, Grass, Psychic, Electric, Dark, Ice... All of them were "Special Types". The only remaining Special Type would be Dragon. Not that that matters anymore, since most of the special types have gained physical moves, and the physical types have some special moves as well. But I you look at the "roots", dragon would be the most possible type of Eeveelution. Maybe there's still Ghost, but I don't know wheter it was a special type or not.
Same goes for the Regis. If they were to announce a new regi, it would probably a dark type, to keep the common weakness towards fighting types. But we weren't talking about the regi's.
Keeping in mind my first theory to be true, a dragon type Eeveelution would have the highest possibility. As mentioned before, a scaly skin and teeth sticking out in a cute way. I hope they will add some characteristics of the Chinese dragon; colorful and looking really mythical.
As for a ghost type... I've always have something in mind like this one:
https://browse.deviantart.com/?q=ghost eevee&order=9&offset=72#/d1mehy1 Only then more Eeveelution-ish in the face. Which means, bigger eyes.