I must say, I find having blood elves as one of the new races a bit dissapointing. They're just too similr to Night Elves in both appearence and architecture. Granted, they are both elves so I guess you'd expect similarities, but that doesn't change the fact that they're disapointing as a new race, since they won't exactly be new.
If this is true and blood elves are going to be one of the new playable races, then chances are the other new playable race will be an existing race as well, but what other existing race could fit with the alliance (assuming horde will get the blood elves)? Goblins, though technicly neutral, are more alligned with the horde (operate thier airships, etc.). Naga definatley aren't alliance worthy (though having underwater citys/zones would be cool). Trolls might work, since they're tribal and they could just make an alliance friendly tribe - likewise for the Centaur. Dragonkin, no. Elementals, no. Demons, no...