The thing about Eevee is, its the adaptability pokémon. That's why Eevee has evolutions like Vaporeon, Flareon, Glaceon, and Leafeon (Jolteon is questionable, but I don't understand why Espeon and Umbreon exist, though I love them both).
Here's the list of types not covered by Eevee evolutions, followed by an analysis of each (Normal has been covered by Eevee, however, this is debatable);
Fighting
Poison
Ground
Flying
Bug
Rock
Ghost
Dragon
Steel
Fighting Type Eevee
Most Fighting types are fighting types because they stand up and punch, with the exception of Hitmonlee and Hitmontop. This may be a possible evolution, but it does not fit with the adaptability concept.
Poison Type
This type is very likely to have an Eevee evolution next generation, because it fits well with the adaptability concept. Also, there are many other pokémon that are only poison types, and have been successful designs. I can't think of any flaws with a poison Eevee evolution.
Ground
A ground type Eevee would be an interesting thing to see, and I think it's very possible. The adaptability concept is questionable, but I think it could happen. There are quite a few pure ground types, and a Ground-type Eevee surely wouldn't too abnormal.
Flying
What? A flying Eevee would certainly be abnormal considering other flying types, and that Eevee has 4 legs, that being uncommon for birds.
Bug
This is a very likely evolution, being both logical to the adaptability concept, and fitting to the other bug type pokemon. The only flaw with this, though, that bug types are usually grouped with grass types, and Leafeon has already taken that spot. Both Leafeon and Bug-Eevee may be extremely identical, and not fitting in the pokémon world.
Rock
A rock evolution would be almost exactly like the ground type, a plausible idea, but this also creates another problem, the same problem of grass and bug. Rock and Ground are even more closely related to each other than bug and grass.
Ghost
Undead Eevee certainly sound interesting, and makes sense in some points, but simultaneously makes little sense. The idea of a pokémon passing away has been touche before, but never as a catchable pokémon (Sheninja is the living shell of a pokémon, so he is not). This is very possible, but it would be very odd.
Dragon
There is no doubt a dragon type Eevee would look amazing, and serve as a great pokémon, but I can't find a single way to link it to the biological side of Eevee evolutions. Like ghost, this would be a great idea, but not coping to the adaptability concept.
Steel
I think a steel Eevee wouldn't be good design at all, and it wouldn't be a very logical evolution to the adaptation concept.