That was my entire point. The only way to make Kul Tiras interesting is to throw away what Kul Tiras is and instead put a bunch of extra stuff that has nothing to do with Kul Tiras there.
A red wasteland with evil orcs is a fairly interesting jumping off point. The fact that Outland is the shattered remains of a another planet is kinda the selling point. Kul Tiras is a tiny island with a human city.
Do you see the difference? Without anything else, Outland could stand alone with that variety and amount of interest. Kul Tiras requires you bring things to it, or change it. So why bother using it as a setting and wasting interesting villains and concepts making that boring setting interesting, when you could instead base the expansion on those villains and concepts themselves.
A lot of things.
An "island hopping" expansion isn't my number one idea, because I feel like we've seen a lot of those enviroments already, (the jungle, the tropical island, etc. ) but it's still a step up from just Kul Tiras, and I'm sure Zandalar would be neat, the Dragon Isles would have a lot of room for interesting content and Undermine would be cool.
Underwater expansions are fairly controversial because of Vashj'ir, but I think one would be fairly interesting, and could be done in a way where the swimming is a small part. It would be neat to see Nazjatar as a zone, and have the massive darkness of the rift just sitting there while you do stuff along its edges.
And as my sig and constantly bringing it up suggest, I think an Underdark/Darklands style expansion would be the most interesting. Caves+Magic means that Blizzard can go
completely crazy, and even without the magic,
plants and animals in caves gives a lot of room for things unlike what we've ever seen and things that would be bizarre on the surface. They can easily imitate any sort of surface terrain (
sandy desert caves,
frozen ice caves,
your classic volcanic lair caves,
Forest caves) and beyond that, such an expansion contains several interesting places of importance we've yet to go (
Undermine,
Azjol Nerub, potentially Nyalotha) as well as
potential civilizations we've never seen or even heard of before. It's a chance for the dev team to just really go wild, and a more fitting place to fight the forces of N'zoth than a human island.