I'd like to see the Reachmen, or 'The Witchmen of High Rock'.
They practice 'beast magics' learnt from the orcs, and are a breton mongrel breed which resemble elves far more than the regular breton.
They seem to resemble what I've always thought the bretons should've looked like, and the title 'Witchmen', along with this beast magic, makes them sound baddass. B)
It'd be a definite possibility to see some of these guys in the reach (they live in the western reach of High Rock), if the next game is in Skyrim, too, which is pretty cool.
They shouldn't be playable though, unless they just add some features as possible addons for the Bretons. I think they should stick to the current playable races myself.