Perhaps the Imp evolution could start off with growing a bit more slender, hair growing longer, and the skin pigmemt taking on a reddish hue? Horns and/or tail could also be an addition.

In some fantasy lore, Imps are often put into the same correlation as goblins, so I see it working. Cunning is one of their most defining traits, I'd imagine.

Of course, I'm just putting out ideas. ^^