There is a moment in Chapter 5, when PC has to dive through the portal really quickly to leave Chult. Now, if you don't, the portal would close. The game is over. Or is it? Would not that have been a sweet optional side-quest to adventure in Chult for a bit and return to Kuldahar by some neat magical means?
Basically, if you have a great idea for a sidequest in Chult (that is not on one hand too "standard" and on another requires creating 20 new areas) - shoot! I can't promise anything, but I *think* it is the coolest missing quest yet.
Well, the second coolest quest is the extension of the Jinni+White Anishai encounter and an underwater sidequest
