For Job identities, I would first look at roles themselves. Figure out what is the minimum required for the Jobs to function in their role and condense that kit.

After having the generic must have buttons for Healers and Tanks, they can go as wild as they want with the rest of the kit.

I'd rather see wild variance in GCD within subroles, someone with more rigid and stable rotation while other has more proc based one (either faster gcd or pseudo random for some badluck protection). Some buildup with spenders, some stabile wailing with alternating phases, some chaining things, some slow big hefty hits etc.

Preferrably most or all DPS would shift from team buffing to greedy DPS so they can more easily balance the vastly different playstyles by purely adjusting numbers without having to worry about interplay with other Jobs. If there's buffs they should be mostly personal or something a Healer or Tank can pop at times if their gameplay doesn't get more complicated.