Hi there,
I'm someone who for a long time has felt that High Elves should be playable or that the Blood Elves should have always been part of the Alliance. When Void Elves were announced, I was disappointed. However, as more stuff was released about them, they seem to have a lot of potential.
At BlizzCon, Ion Hazzikostas said High Elves wouldn't make sense given their small numbers. And that's a fair point. Yet Void Elves were officially described as a small crack group of Elves, if I recall correctly. Their numbers are probably no more, if not less than, the High Elves. So it doesn't make sense that they'd be an allied race going by Ion's logic.
But yes, Void Elves wouldn't really feel established (lorewise) as a race if they're extremely small in numbers.
Here's a crazy idea for them to have significant numbers. And I'd like to emphasize that nothing is set in stone, and that in a fictional world the writer can pull anything out their ass.
There could be a series of events in Quel'Thalas that leads to some sort of powerful Void entity or energies that somehow results in it bringing back many or all of the fallen High Elves (from when Arthas pillaged Quel'Thalas) back to actual life across the land, however, as a result of them being brought back from oblivion via void powers, they are now permanently Void Elves. In which the Blood Elves drive them out because they don't want them there, and they go join the rest of the Void Elves in the Alliance.
This would give them a massive population boost, and make them feel like an actual and established race.
Thoughts?
TL;DR: Have an event of sorts where all the High Elves slain by Arthas' rampage are resurrected via void powers, which also transforms them into Void Elves, and have them join the Alliance.