I've said this before, but if the race has been allied with the Alliance or Horde in the past, they SHOULD be playable. People can argue and say races like High Elves, Wildhammer Dwarves, Broken, Furbolg, Jinyu, Ogres, Taunka, Forest Trolls, Undead Elves, Hozen, etcetera are mere "flavor NPCs", but you can't take away the fact that they play a role in their respective faction's story, no matter how minor a role they may play.
Allied Races are chosen based on their relevance in the ongoing narrative. If they ever revamp Azeroth, these already established "allied races" can be made playable and they should. People are always going on about Blood Elves looking identical to High Elves, which is blatantly true. However, the story has emphasized a political difference between the two, just like the Tushui and Huojin Pandaren. Also, people say there's no way to make High Elves different from Blood Elves. That's preposterous. Look at all forms of media you consume; they all have stories, stories that are constantly evolving.
Who's says in the future Blizzard can't write the High Elves differently? All that's needed is a story that has Alleria and Vereesa Windrunner coming together to unify their scattered people. Cliche, I know, but it can be done. Then, from there they can give playable High Elves a slightly different appearance, after all Void Elves are really only different from Blood Elves because of their blue/purple skin, their void powers, and occasional tentacles. They also gave the Void Elves slightly different hairstyles, and remember, Void Elves are a flavor of Blood Elf. You essentially apply this to High Elves, give them some tattoos, and boom, you've got a slightly unique Allied Race.
The main point of all this is that more options equals a more diverse and immersive world. We're all entitled to our own opinions, after all and everybody's is valid. Honestly, I'll take High Elves in any capacity (my main is a Blood Elf, so spare me the "play Horde" comments), whether they are made into an Allied Race or added as Void Elf customization options; we don't know how the next generation of Void Elves are going to be made anyway. Once again, for me, I welcome any new playable races or customization options because it enhances the world and the overall story. You can never have too many options, right?
P.S. Grimtotem are a split people - there are those allied with Magatha and those allied with the Horde under the leadership of Jevan "Stormsong" Grimtotem. Many people tend to forget this important piece of information. Source: The Shattering: Prelude to Cataclysm. And yes, they are originally from Mulgore, we all know that.