Oh, please, no new races. TES has 10 playable races and that is quite enough, I think. If the devs try to make them as they are described in the lore this time it would be great. Seeing some lore supported races as UNplayable could be nice though.
I agree, unplayable Tsaesci, Sload, Falmer, and maybe even some Umbrielan races would be interesting (but that depends on book two, really). As for the current ten races, don't add new ones, MAYBE make subraces for Khajiit, and make them more distinctive both visually and in terms of gameplay (after all, old cultural hatreds die hard).