It'd be neat to add alternate paths to some (not all) dungeons similar to how Bravely Default did it. Throw in 3 paths - 1 dangerous / short path, 1 middle path and 1 long but easy path. You could go through the dangerous path, only if you had a very coordinated team, and it would get you to the boss the fastest. The middle path would be like what we have now. The long path would be lots of running, but the trash would be easy to kill. This may be the best path if you have swiftsong :P

I'd like to see more dungeons similar to Haukke Manor where it's not a linear path, but more rooms within the manor. You follow a set path, but the dungeon itself is not a set path.

Although I like the trinity system, it would be cool to see a dungeon designed / balanced around any combination of 4 players. This would be less combat focused and maybe more puzzle / timing focused.