Giving DF a bit of leeway here, since the connection between all the MoP races were not immediately apparent until 5.2.
Still though, you are right. Most of the races we encounter on the Dragon Isles feel like they were chosen arbitrarily, rather than as a natural development of the world.
The Niffen, being a new thing rather than a preexisting cavern dwelling race is one thing. Another is how Centaur are torturously forced into the plot for no better reason than there just being their turn to get an HD upgrade. The Tuskarr are slightly better, but only because of the proximity to Northrend.
I have to wonder if this really is just down to the developers continuously upgrading old models and needing to find reasons to use them.
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I would say it's more like you have one really solid core concept of South Seas expansion with Pirates, Naga, and an Old God undercurrent. Being invaded by a poorly thought out Faction War one.
I wonder if I'll ever be as disappointed by an expansion as I was by BfA. On paper that expansion should have been an 11/10.
But anyways, we should probably get back to Dragonflight lore. Sadly there isn't much we can do to make BfA better st this point.