I'd second that poison one, I think it'd go real well, especially considering that the last 2 times eeveelutions were introduced, one was weak to the other (espeon -> umbreon, leafeon -> glaceon). It would fit the pattern.

Eeveelutions have also been special types. As in the pre Gen 4 physical/special split. So realistically, the only other eeveelution I'd expect to see is dragon.

>>26394389
I think it should be fighting. It would fit into dark-psychic combo to complete the circle of weakness. Or maybe ghost type (evolves when all your team died except eevee when it lvls up or some shit like that).
Anyway, I doubt it will top sylveon. After I got mine in gen 6 i fucking loved it. I evolved it just to see the whole fairy type but soon it become my number one pokemon in team up until the very end. And that the 1st time any eeveelution stayed in my team through the game.

>>26394389
Bug would be fun if it gets good bulk and a slow U-Turn. Could be a good Wish passer that damages the opponent as well.

Remember, whenever a new eeveelution is introduced, so is a new method of evolution.

Umbreon and Espeon showcased day/night evolutions. Leafeon and Glaceon evolved via proximity. Sylveon via Amie.

So when you have a new eeveelution idea, it has to come with a new evolution concept.

>>26394413
If you want to go with patterns it would be dragon. Every single eeveelution so far has been for a type that was consider special pre gen IV, with dragon being the only unrepresented one.

>>26395035
Honestly expected Dragon to match Sylveon in Gen VI. Missed opportunity, I guess, but then they'd have to make a way to anger your Pokemon in Amie.

I want another trio, and I want Flying/Fighting/Poison, I think it would round out the balance quite nicely.