-Some "expanded dungeon" system that uses alternate less hallway-like versions of existing dungeons, not part of roulettes, that have weekly cooldowns for unique rewards and require pre-made parties who can have minimal coordination. Maybe take inspiration in the abandoned bonus chests system that Yoshi-P had added in 1.X with the bonus goals being randomized for each instance. Maybe try to balance the difficulty of combat in them to be between final MSQ dungeons and Extremes or something.
-More dungeons in general. As a pve-focused player Shadowbringers's dungeon cycle has been pretty bad to me
-Next Not-Diadem not following Bozja's progression or fate scaling system
-Hamlet Defence Mark II
-More features in the overworld, maybe an Eorzea Archive that you expand by clearing existing and upcoming sidequests or interacting with things in the maps to encourage exploring points of interest
-New areas that don't have massive chunks of nothing when explored up close and aren't just enormous arenas with aggressive monsters all over the place with no rhyme or reason
-Give tanks, healers and I daresay some of the DPS more identity back