Ghost never really made sense to me as a Physical move anyway. I even forgot Shadow Ball was physical when I taught it to my Mewtwo in FireRed. However, I think Dragon is most likely, and I would be glad if a Ghost-type Eeveelution existed too.
I'd definitely say Dragon. All of the Eeveelutions so far have been of the "special" types: Fire (Flareon), Water (Vaporeon), Grass (Leafeon), Electric (Jolteon), Ice (Glaceon), Psychic (Espeon), and Dark (Umbreon). What's the Special type that has not been "Eeveelutionized"? Dragon.
And regarding this Dragon-type Eeveelution, thinking of most Dragon-types, as well as the trends in the Eeveelutions' stats, I'd predict it to have base 130 Attack.
I wouldn't mind some new Eeveelutions, although I must admit it's not on top of the list of things I want the most. Considering that I didn't think Leafeon and Glaceon was all that bad, things could end up pretty decent.
As for type, just to go against all that says Dragon, I must say I can't imagine a Dragon-type Eeveelution, special-type or not. I'd like a Steel-type actually.
Well, Eeveelutions are okay, but I don't like to see more of them anytime soon, a little break would be nice (like in Gen III). I also can't really imagine anymore suitable types for them, dragon-eon sounds... not Eeveelution for me.
Umm, if there's going to be a new Eeveelution, I hope it's a ghost-eon. It could be nice addition :3.
Amyway, as much as I'd love a Ghost and a Flying type Eveelution, I don't really think they'll make more.
If they do (and that's a big "if") then Dragon would be the only type I see them doing due to the "special types" pattern they've been doing (even in Gen IV. Both Leafeon's and Glaceon's types used to be Special). Although they could have made a Dragon type in Gen IV to make a trio like in Gen IV and yet they didn't. Perhaps we'll never see a Dragon Eveelution. Which probably means we won't see any more, period.
Maybe a Normal type level up evolution to pair up with the Dragon one, but even that is very unlikely.
3) To those who state that enough is enough on the Eeveelutions, I present this interesting thought for your consideration. Which generation had no new Eeveelutions, and which generation do people generally like the least? The answer, for me at least, is the same for both questions: Generation III. I also do not consider this a coincidence.
4) Finally, there are so many items from previous generations that aren't used very much for evolutions. Since you won't be able to trade any Gen V Pokémon back to any of the previous generations anyway, least of all Gens I, II, and even III (with the GBA slot disappearing on the DSi), why not find new uses for the old items? And why not use that to facilitate an Flying or Steel Eeveelution?
You know what I never see suggested? A third stage. No one seems to realize that every Eevee line so far has room for one more evolution. While I doubt Game Freak would do it, they certainly could if they wanted to, and most of the element-based Eevee evolutions certainly have room for "improvement".