Shadow Hunter is probably the last unique bastion. It involves Loa and Voodoo which if you want in a in-game pretence is like Paladins with Tank, Damage and Healing but instead of in the Light and Holy identity, they use more Shadow base with a strong Voodoo identity, which for now can be stitched with together by the Loa and connect it with the Shadowlands or even the same meaningless place Shadow Priests had before the Void involvement and no necessary background that has to get involved. They could even throw a dumb curveball that Vol'jin being in the Shadowlands in death can pass on his knowledge to be a Shadow Hunter, with enough work and logic it can become something like Blizzard pull many things out of thin air.
None of the classes specifically stipulate their Shadow based magic is from the Shadowlands at the moment. Death Knights use Death and Necromancy which can stem from the ideals of Shadowlands (thanks to Chronicles and their pretty pictures), but that's quite a small section really never built upon because the realms of Frost, Blood and Unholy waver and the lack of acknowledgement although really which can work in a new class's favour. Death Knights are also are quite militant in they need rule, leadership and a almost agnostic approach needed.
Shadow Hunting is completely the opposite as it is a religion funnily, they worship several Loa who bless their followers in capabilities to be melee, ranged and uniquely all attuned together with healing abilities. Priests has always been an odd fish in that really, they were two forms pushed together with no real flow within to say why Shaodw and Light can stick together without things combusting per say and never really built upon, whereas Shadow Hunters kept their archaic ways and can master it through the different Loa present, with a prayer they can become one and all.
All in all, the Shadow magic area has Shadow priests and Death Knights but the fantasy is pretty much unidentified or moved away entirely.