Since the next expansion will be called "Dragonflight", the only new class I could think about is Dragonsworn.
New classes have always be related to the expansion they stem from. I'm sorry Tinker lovers, but your class is not gonna happen (yet).
That being said, how do you think Dragonsworn class would be? I don't think they're gonna give us more than 4 specs, so two flighs will probably be subsumed under one spec (red and green under the healer spec?). But besides that, what other features do you think it will have? Morover, what would you like to see?