-How hard would it be to try out an expansion
-where zones aren't locked by MSQ,
I can understand this from a development perspective seeing as how crucial the story is to the game. Now, what I could concede on is...
-Where content is not locked behind story progression.

-where quests don't have braindead markers telling you where to go
Honestly, I don't mind this, what I do mind is that every zone is so small, is its own instance and flying. This makes the world much smaller than it should be.

-where dungeons are nothing like the stale formula we have
Agreed, though the only caveat here is that dungeons are a stale formula because it is essentially how people ended up playing the game.

-where underwater isn't just a pretty landscape and combat/breathing mechanics are tried out.
Would love this.

-Where exploration and open world events are tried through the entire introduced zones instead of designated exploration instances like Eureka and Bozja.
This is one of the biggest things I hate about Eureka/Bozja, it's a constant prompter for what the open world zones should be at a bare minimum. Eureka and Bozja should not be content. It should be embedded all throughout the game.

-Where some side quests are just damn hard
Would be nice if the solo instances had a little more engagement.

-Where some open world dungeons are introduced, and the mobs are just lethal
Agreed, but I think hunts should be reworked and more normalized on a much smaller respawn timer. I am probably looking at something akin to how the OG OG Diadem was, way back when it was ancient and combat oriented, and with the emergency mission. I know people feel strongly but I think OG Diadem was just ahead of its time.