Personally I loved Vash'jir, though I know most people didn't for various reasons. But I think there's still a lot of potential in underwater/deep sea zones like this, and I think something really great could be made if Blizzard took another go at it, and looked to what the players had to say about Vash'jir.
So yeah, if there were another zone like this, how would you like to see it differ from Vash'jir? Or what would you like to be kept from before?
Personally I'd like a wider variety of mounts, more giant sea creature enemies (and perhaps tameable hunter pets for use in the zone too?), and gear with a very heavy underwater theme!