Circa 2007 calling.
I think we will get a new class eventually although I don't expect it for atleast another expansion of two after BFA. Allied races will be a priority for the moment I imagine because they add lots of character variety without having to think too much about balancing a whole new set of abilities.
Speciation Is Gradual
I'm well aware that class halls are more specific than a generalized garrison. But the point is, they'd need to do something, whether it's tie it to a similar-themed garrison (IE, Necromancer to the Death Knight hall) or just throw up a generic mission board in Dalaran for them to use so they can at least complete the bare-bones mission board junk for various questlines.