I like the suggested ideas to give a more randomized approach to the deep dungeon system to be more roguelike.
They could give give all jobs a pvp-style specialized skillset with many additional learned abilities you can slot in like BLU.

Although I doubt they will put any effort into new stuff, given they just copy/paste ideas like criterion and call it a success... - same core with different paint seems to be the name of the game now

Aesthetically a tropical themed deep dungeon where you start at shallow beachy areas and work your way down to deep-sea-esque floors with outlandish creatures would fit the new expansion imo while giving a sense of really delving into the depths of the ocean to find treasure