(Gosh darnit! The back button on my mouse is too sensitive. I had to do this all over again -___-)
Despite the pattern of the typing, let's say it wasn't going to be a dragon type (and it most likely will be).
We can pretty much toss out a ghost-type evo. Why? Because all of the eeveelutions (including eevee) can learn Shadow Ball--even the offense-minded ones. I also doubt a bug-type, because the Special Attackers (Glaceon, Espeon, Vaporeon, and Jolteon) can all learn Signal Beam (but there's nothing barring a physical bug type).
Remember, Eeveelutions are not meant to be flexible or versatile. They're meant to excel at two areas with one type and one type only (Vaporeon maybe being an exception, with a strong Ice-Type move at its disposal). Dragon-type, of course, is most likely at this point. But what about the stats? Will the most likely spread support a typical dragon-type movepool? Let's take a looksee, shall we?
All Eeveelutions have a 60/65/65/95/110/130 spread. (And a completely balanced evo is both likely and unlikely. If the next evo ends up being the last, I don't fully doubt a balanced evo. But if there's more to come, I don't expect them to break the pattern anytime soon.) We can take a look at which Eevee evos command (and don't command) each stat.
HP: Vaporeon has 130, Umbreon 95. The rest have 65.
Attack: Flareon holds the 130 spot, and Leafeon has 110. The rest are 60 or 65.
Defense: Leafeon has 130. Umbreon and Glaceon have 110, and the rest are 60 (my god, that's pitiful o_0).
Special Attack: Glaceon and Espeon have 130, Jolteon and Vaporeon 110. Flareon has 95.
Special Defense: Umbreon has 130. Flareon has 110, and the rest (excluding Leafeon) have 95. Leafeon has 65.
Speed: Jolteon has 130, Espeon has 110. Leafeon has 95, and the rest are 65.
With this distribution, I think we can rule out another special attacker. Out of the types we have left (Bug, Dragon, Fighting, Flying, Ghost, Ground, Poison, Rock, and Steel) most are Physical attack-minded. Dragon type is the exception (there are only three physical dragon moves).
Ghost I already ruled out. Bug is viable if its going to be a physical attack-minded stat distribution. Ground I doubt due to the fact they can all learn Dig, but its still possible. Flying is certainly possible, seeing as we still don't have a pure flying type and only Leafeon can learn a flying move (Aerial Ace).
Truth be told, it could be any of the remaining types. Heck, if it ends up being the last evo, I don't doubt a normal type evo. (130 Attack and 110 speed would kick butt, though a normal evo would be the most likely candidate for a balanced stat distribution.)
But another we have to keep in mind, other than the technical aspects, is that an Eevee evolves for the same reason animals do: to adapt. Vaporeon was a result of an eevee being around Water. Jolteon from being exposed to electricity, Flareon to extremely warm temperatures. Espeon and Umbreon evolved from the exposure to sunlight and moonlight, respectively. Leafeon was from living in a forest, and Glaceon from living in the cold.
Bugs live in forests, but Leafeon covers that. Dragon types... I don't know what you would need to evolve into that. Fighting may result from living in a harsh area. I would say mountains, but then what makes Fighting different from Steel or Rock? Either one of those could from the exposure to mountains, so which one would it be? Flying makes the most sense, if an eevee is forced to dwell in the air. Poison could come from too much exposure to toxic conditions. Maybe an Eevee spent too much time in the Underground and evolved into a ground type. As for ghost... it would have something to do with death.
We'll just have to see ^^ I'll keep holding out for a flying or dragon evo, though, no matter my reasoning.
Edit: (Jeez. That was all over the place xD)