First, let's establish I really don't want this to spiral into a "GAH WORGEN SUCK" or "I HATE BLOOD ELVES REMOVE THEM" thread. What I'd like to know is, what races have you bumped in to (via reading or in game) and gone "Hey, I'd play that if it were available." Also, why?
A race I'd love to be added would be the Vrykul. I love their animations and lore. I couldn't give you a logical reasoning why they'd become playable, so I very much doubt we ever see it happen.
One I would be interested in seeing (maybe not playing, myself) would be the high elves. They've always been mentioned, but lack an actual unique model... either it's a Night Elf with certain color eyes (I can't remember if it's gold or white) or Blood Elves with blue eyes and a Night Elf voice. I could see them getting integrated into the game as another "pick your faction" race though, not something I was a huge fan of.