He fleshed it out by adding in unrelated more interesting content. That is kinda the problem at hand, why even make it Kul Tiras?
A group of unpopulated islands would be interesting if the Naga and Void and Old Gods and Scourge set up shop there. If you have to import 3-4 unrelated threats to a region to make it interesting, you should probably just choose a region more suited to those threats. The Broken Isles carried itself because it was the location of the Tomb of Sargeras, that alone made it noteworthy. Kul Tiras is the location of a now incredibly small and minor naval kingdom that hasn't been relevant to most of the world for decades.
Hence why, when people make fan-concepts of a Kul Tiras expansion, it isn't actually about Kul Tiras, its about more interesting things coming to and completely changing Kul Tiras.
It's dumb and I really hope Blizzard isn't going to do it rather than actually use the locations suited to N'zoth and Azshara, which tend to be way more unique and interesting than generic continental habitats.
e.g.
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I have to hope the Dev team would rather make full use of the ability to do crazy, interesting locations rather than force themselves through another expansion of regular forests and mountains and coasts and valleys.