There is few problems wit that:
- You are introducing 3rd playable faction
- Those 3 specializations are quiet frankly impossible ( dragons don't change color just like that, for proof do questchain in coldarra)
- That idea is not new it is old some 2 years and I remember that there was more detail at that time in it.
- Lorewise they would not fit in no matter what.
- What race would they be? Elves, Humans, Orcs... Why would they swear allegiance to some dragonflight?
- Would they be Dragonkin.
That is about it, personaly, I think that would never work at least not in Warcraft universe.