A ghost-type would be really cool, one with legs that fade so the paws cannot be seen, and the colour scheme would be blueish purple with black eyes... And it would look like this sad little Umbreon that someone deserted and it died out of loneliness...
And I would really like to see a dragon-type eeveelution. It could have these small horns and wings and even longer tail than what Vaporeon has. I guess the colour scheme, being dragonish, would have to be something dark, too. Like dark red and navy blue or something. The secret in that one would be making it look cool but still maintain the cuteness essential to eeveelutions.
Now that you mention it, a steel-type would also be nice. I imagine it would look somewhat absolish, with the same sort of habitus and the colours somewhat alike, too. A steel gray fox-like creature with some sort of protecor plates made of metal on it's body. It would have a metallic cry, too.
A poison type could be cool, too... It could have fangs xD Or poison spikes on it's tail or something. It could be greenish...
Man, I really want to work as a Pokémon designer :D it would be a loss for the world, but a win for me! And now I really want to make pseudo-eeveelution sprites...