Miscellaneous Adjustments
Debuffs
While extremely controversial, debuffs such as piercing, blunt, and slashing should make a return. As of Dawntrail there are currently 21 jobs and more on the way, by bringing back debuff it would lead to multiple meta party compositions. The meta would be able to be changed per fight basis as SE could make some encounters immune to certain debuffs.
Aggro Management
The current iteration of aggro management, it is effectively a non-mechanic. Aggro is a mechanism that should be managed by all roles. Stormblood had the best iteration of the mechanic so I will be building from that.
While it was difficult to maintain aggro for some tanks this can be remedied by reintroducing an equivalent of Sword and Shield Oath. By adding a 30s cooldown that would increase aggro generation would also add another form of aggro management for 2 tank fights
For Healers enmity dumping was removed from lucid dreaming making it a skill only used when you need mana. Adding a separate skill for aggro dumping or reintroducing it to lucid dreaming could work.
DPS aggro management was also completely removed. Reintroduce this by various means (Diversion or Quelling Strikes) or similar to healer management.
Meta Compositions
Meta should be embraced as it allows people to feel accomplished as they figure out how to optimize an encounter. All Classes should still be able to complete an encounter, the meta would be able to complete it faster.