I really feel both the game and the narrative would be most enjoyable if they instead told the story of he various races meeting new challenges and threas ratehr than individual characters or classes or faction.
Faction is too boring, it just feels like we're two races and hate is the basis of all things - no fun at all
Swtor based theirs on class - but class lore isn't that deeloped in wow, besides, classes are just the program to interact in the game space - Legion class emphasis would have been more meaningful as races.
Now races is where all the fun and depth is. Your tribe, your people - for an anonymous champion, your race represents your identity in a far more meaningful fashion imo.
Sadly there are too many races to carry out meaningful campaign, even if you fold allied races into their main race. However I feel there is a thematic grouping that can work well as mega campaigns which can have minor vraitions and additions for the individual races: I would have 6 majoar campaigns.
Orcs, Trolls, Tauren , Goblins - themed
Humans, Thalassians, Dwarves, Gnomes - themed
Undead of every race, scourge, Ebon blade
Night elves, Nightborne, Worgen, Illidari, naga
Pandaren, Mogu, Jinyu, Hozen
Draenei, Lightforged, Eredar, Broken
Now obviously everyone doesn't need to be friends, and there are interactions across the various campaigns, a race might have it's primary involvement in its native campaign but also have secondary involvements in other campaigns due to its relationship and connecions with a race in another campaign.
But these are the primary world focuses.
Well, however ou divide it, this is what would be best