Suggestion : Give each job two distincts "purpose" to build on future customization

[Reynhart]

Disclaimer : I'll mainly take tanks as example because it's the role I play the most, but I'm sure we can do the same with each job

Character custimization is, in my opinion, a big lack in the game. Apart from your gear, you're just the same as everybody else, when it comes to your job.
The reason why they don't want to include it is, officialy, the fear of increasing discrimination based on that. And people tend to agree saying that there will always be an optimal way, so what's the point of adding another.

My answer : give each jobs several purpose

The easiest example is for tanks. Most of the time, in 8-man content, you have a Main Tank and an Off Tank. And it's common sense that those two don't need the same thing. Thus, wether you prefer playing as a MT or an OT, maybe you'd like to build your character towards it, and you could have a skill tree where one branch gives more edge to MT, and one, to OT.

For example, as a MT, you could use traits to increase the enmity bonus on Savage Blade, or even adding one on Royal Authority. You could also gain a trait to allow Shield to block magic attacks, which is mostly useless for an OT. This way, even if people theorycraft an optimal way for each role, you'll still end with twice the type of tanks.
Of course, you could end in Duty Finder with two tanks specialized in the same role, but "not being optimal doesn't" mean "can't clear the content", and you can already end with two PLD or two WHM, thus having a not optimal setup, without any real issue in the end.

For healers, maybe you could split between single target traits and multiple targets...for DPS, burst damage vs sustain damage, maybe ?