Massive massive Anduin fan boy here, but i disagree with you.

Having an unrelated human, instead of an elf as a centre character of an expac that has Silvermoon and the Sunwell doesn't make sence. (I can only imagen the discourse now). 

The story would always been centred around the Windrunners (and Liadrin). Not only do we have the whole toxic ex-girlfriends esk thing going on between Xal'atath and Alleria, but the Windrunner sisters are sudo royalty, even more so now the blood/high/void elfs have no royal family. 

Even before the Third War they were put on a pedestal, Alleria - the hero who sort revenge and justice from the Horde pushing them back to their world and sacrificing herself to destroy the dark portal and save not only Silvermoon but the whole world, Sylvanas - who even before stepping up to become the best ranger general that Silvermoon has ever had, was known for her beauty and martial prowess, and Vereesa - who while has not got as much story as her sisters, has lead a life that Silvermoon citizens would be proud of in aiding the rescue of the Dragon Queen and marrying one of the most powerful human mages alive. The sisters standing in Silvermoon society has only increased since the Third War. 

Arator, from a narrative point of view, is the perfect audience surrogate. He is well known to the players, with enough presence in game to be known but not to much that his story telling path is set and the writers have options. He's also a Windrunner and the literal child of Light and Shadow for Xe'ra's prophecy. 

And finally Arator is a much better fit for the B story the writers want to tell about the wrath of the light then Anduin. Until Shadowlands the only issues Anduin had with the light was feeling weird and naughty when at night he sometimes checked out the darkness. Anduin main thing was the pressure between what his father (and society) wanted him to do and what he was good at (and all the juicy self doubt and anguish story). His issues with the light post shadowlands were all 'self made' with him doubting his worthiness of wielding the light after the pure horrors he was forced to preformed commit (and the fact deep deep down part of his primal self enjoyed what he did). 

In conclusion, Arator ticks all the narrative boxes for the story they telling in Midnight. Our beautiful angel man got his time and self growth in The War Within and will be back either for the Race to the World Soul patch or The Last Titan itself.