Some potential ideas:

Stance-changing Healers
Splitting healers into pure and shield was a bit of a mistake, and I feel the solution is to look at Astro from 3.X and its ability to choose whether it'll be a regen or shield healer at the start of a fight (you would activate Diurnal or Nocturnal Sect before combat and it would lock in once fighting starts). Implementing this for the four healers where a stance augments their kit toward pure or shield healing would be really handy as instead of having to search out one pure & one shield (or two shield) for PF you can instead take any healer and just have them adopt the relevant stance. Would be an opportunity as well to add some aesthetic to each job; a White Mage could have an Earth theme on its spells when being a shield healer, and the current Water theme when it's doing pure healing.


DPS Swappable Skills
Some old playstyles were lost to job reworks that have left some players without the job playstyle they enjoyed; Summoners are probably the biggest example, going from a DoT management style to the current system that's been criticised as being overly simple and a bit rigid to play around different timings in phase changes. However, there will be players who came into those jobs not knowing the previous playstyle so just reverting them would be a bad idea.

A way of restoring old playstyles could be through swappable skills on the role-action system. In the past, this system was used in a way where you'd pick a number of skills from the ones available to equip and you would take whichever ones best fit the fight(s) you were planning to do; for this system, it'd be picking one of two conflicting skills that change the playstyle of the job.

As an example, let's say you could either equip DRK's Living Shadow (does damage independently of you) or Dark Arts (buffs potencies of certain abilities while its active); the damage from Living Shadow should be comparable to what you get from using Dark Arts appropriately so that the playstyle can be changed without overly changing the job's potential damage too much; one style would be better in certain situations vs. the other, but a player could change it between pulls to better fit what they're dealing with if an issue comes up. It'd be a way of potentially getting some old styles back that sections of players weren't happy about losing, like SAM's Kaiten.