Originally Posted by
Arridor
There is a difference between "it doesn't fit logically" and "I don't like it". You're thinking of the latter.
Storywise, Worgen joining the Alliance make sense. Goblins joining the Horde made sense long before that. It's Draenei and Blood Elves' reasons to join their respective factions that are rather iffy. Sure, blood elves had Sylvanas "invite" them, but then draenei joined because... they have nothing in common with the faction but similar religion shared with one of the races. It's like Morocco joining NATO because they're Muslims and the French are Christians.
Gameplay wise, in all of those expansions a new race was always a fresh addition. If that depended on me, we'd have like four new races every expansion, just to have a lot of variety.
Now, personally, I think prime candidates would be three races. Ogres for Horde and neutral Ethereals as player favourites, while Alliance would get Arakkoa as a counter-point to Ogres.