This is 1000% unrealistic and would take up too much resources, would be even unbalanced most likely.

But I'd add specs to classes (Such as DPS warrior, Healer pld, tank Samurai ect).
Obviously this would take so much effort for something that's really unbalanced, but I know I'd enjoy it even with how unbalanced it would be.

Anything else I could add outside this
>Selectable role actions. (Not like stormblood) but where you would select 1 out of 3 similar actions in one category (such as rampart with 3 variations).
>reworked melding, to be more interesting
>Add raid buffs that aren't 120

In general I just would try to make the job design direction go into a different place, current job design feels watered down and bland and is my main issue with the current game, I want them to take more risks with job design as that's more *fun*