I think ground, flying, and normal are the most probable. I would love to see ghost however, that is my most anticipated eeveelution, and the second is bug. Although, I'm understanding the argument that says eevee only evolves into types that represent an environment... but then I look at espeon. I know that espeon is the daylight pokemon and umbreon is the night time pokemon, but what I dont get is... why psychic for daylight? That just doesn't make sense to me. I know they needed a way to counter umbreons dark type, which actually made sense. But the way espeon evolves just doesn't make sense to me, it sounds like they almost just threw espeon in for the sole purpose of having a counterpart to umbreon. I don't understand why a psychic pokemon needs the daylight and how that would fit as its environment.
Basically, what I'm saying is, if GameFreak really wanted to, they could probably just make up an environment for any type that they wished and it doesn't even entirely have to make complete sense. For example, I could see a dragon type eevee living in the mountains just as I could rock, ground, steel, or fighting, or a bug type eevee living in the trees as well as flying. Poison could be in sewers, polluted, or marshlike places, and ghost could be lonely environments such as abandoned houses or creepy places you normally find ghost pokemon (maybe an eeveelution that evolves at a level up with 0 happiness? I could see that!)
But then you see espeon. What kind of environment could you think of for a psychic type pokemon? I honestly cant really think of one.. all psychic pokemon don't really have their own place, they're usually just found in grass at random spots like drowzee or in the mix with ghost types. Daylight and psychic doesn't make sense, so I believe that this proves that there can be an environment made up for any type, leaving the possibility that ANY type that hasn't been used yet has a possibility of having it's own eeveelution.
The only thing that does bother me however, is seeing eevee change from a mammal to a reptile or bug. I believe that this can be easily countered as well however, as there are pokemon that represent types that don't even look like the type they represent. Take lucario and steel for example, or dunsparce and normal (looks more like bug/ground to me), or especially gligar and gliscor being a ground/flying type which to everyone looks like a poison/flying/or bug type (it even has poison heal for crying out loud!)
Basically, a steel eevee might not have a steel coating anywhere on its body, a dragon eevee might not have to look reptilian to be a dragon, and a bug eevee might not have to have an exoskeleton look to it to be a bug. The possibilities are endless! It is never too late for GameFreak to change things up a bit and make eevee evolve into something different, or hey, even add a new light typing. They converted magnemite to a steel type in second gen, they could convert a lot more Pokemon to light type in 6th gen to balance it. A new type is a whole different discussion though.
TL;DR: I believe that every type has a chance at having its own eeveelution.