High-Elves are in the Alliance since thousands of years - even if they splinted, even if their population in the Alliance have been strongly reduced, they had many time to breed since, and now, we can see in the game that there is MANY MANY high-elves, perfectly integrated into the Alliance, they fight for the Alliance, they are almost considered like humans, they also played an important role in the game, and in the Alliance - they learnt the magic to the Alliance, also, we know that in the next expansion, Alleria will be "back" ("" cause we never seen her in World of Warcraft), Alleria which is an important char in the Alliance since she led the Alliance expedition in Outland, isn't it a great leader for High-Elves ? Also, it's a good way to make some interesting storrytelling about her and Sylvanas who is an Horde leader.
Also, most of the class could be playable by High-elves :
And eventually Warlock . (Have to think about it, it's a bit tricky cause we never heard about High-Elves warlocks)
Also, High-Elves already have a racial mount which is the Quel'Dorei steed (http://images2.wikia.nocookie.net/__...orei_Steed.jpg), Blizzard could easily make some racial mounts for High-Elves with this. Even if it's a horse, it's totally different from human's horses.
Plus, elves are really appreciated by players, High-Elves would attract elfs players in the Alliance, and it is a way to balance the Alliance-Horde population, which is at the moment overwhelmingly horde (except on some servs like mine, i mean, generally).
It seems way more conceivable and reasonable than some of players fantasies like Nagas and Ethereals ...