Sylvanas & Tyrande are both half/half combinations of Hunter & Priest, or considering her W3 abilities, Druid. And that's even before Danuser's whole "Heroes can do more than player classes because Anduin gets plate armor for some reason" and Swan also appears in the Maldraxxus Maw Assault as a companion: Those companions feature interesting mechanics including a tank alchemist who transforms into a sludge monster & Swan's sonic attacks.
I agree that Dragonflight, Dark Ranger, Tinker & Bard are probably their entire shortlist for new classes, but now that Shadowlands is over, I think the game needs an anti-parse supportive DPS class than it needs "Another DPS who uses a Bow"
And "not really any that use music for combat" is the same criticism you can leverage at D&D Bards. That doesn't stop them from being really popular.