An avian playable race that can fly on under its own power? That will never happen. The entire playerbase would be in an uproar because their favorite race can't fly whilst the new one can. Not to mention it would require the creation of a new skeleton, new character movement physics, etc. Simply put, it would be a huge amount of work with little payoff other than complaints for SE. Instead, I think "mounts" that are worn by the players instead of ridden could be an interesting next step. Angel wings, Fairy wings, bat wings, light wings (like the achievement pony), and a jet pack are some options that could be explored here. I doubt something like this would happen either, but at least it's something that wouldn't polarize the playerbase.