To be fair, Demon Hunters had a niche once metamorphosis was decoupled from Warlocks. Once that happened, you had a melee fighter with demonic abilities that could transform into a demon. That gave it an open niche. Blizzard would have to decouple undead summoning and various abilities from the Death Knight to do the same. It's important to note that unlike Warlocks and Metamorphosis, the ability to summon undead has been part of the DK concept since the beginning.
Do you feel that Necromancer didn't fit the setting of Shadowlands?So what class is fit to take the spot? Whichever Blizzard feel is the coolest to add based on the expansion theme. DK totally beat out the competition. Monk simply makes sense as the coolest one that fits the setting, and had little competition considering the expansion theme. Demon Hunter got picked, even if Wardens and Priestess of the Moon had direct connections to Broken Isles through Vault of the Wardens and the various Temples of Elune. DH was the cooler option.
And which setting will we explore on Azeroth next, after Shadowlands is done? It will be what they consider to be the coolest idea they have at the moment that fits the story they want to tell next. And that is the big question - where does the story go from here, and what is the coolest way to explain that story.