I think customization can only work with pvp. We have the option to customize out abilities from 6 other classes(2 if on jobs) but it becomes less of a customization and more of a Needization. Meaning the abilities we set aren't really allowing us to "customize" our character but rather setting skills we need to be optimized.

Take BLM for example or any job class which has archer as a skill set. They NEED to lv that class to 42 to obtain chameleon to assist in hate control.

I mentioned this idea back in another topic but I honestly thing the best method of customization could be learning BLUE magic which can be used only in PVP. At least then you don't have to think about balance much, every player can attach up to 5 BLU spells and they have 0/x points to work with. So there could be 50-100 BLU spells in the game which mean you would be able to give yourself a nice bit of customization to keep your foe on their toes during a match since a Warrior could attach healing abilities to them to make them healer to catch a group off-guard thinking that the two warriors have no healing support or a BLM could run into the middle of the battle-field with an almost drained MP pool but have BLU spells set to make them more durable and take hits to draw out the enemy.

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When it comes to customization, while we all would like the option to give ourselves unique skills/aspects to set ourselves from others, in a PVE mentality it just doesn't work because their are always "god" combos that will have to be used rather than whatever you set yourself as because it's already been proven to work. Then of course by adding more abilities for customization it just takes that much longer to balance PVE aspects so by keeping it simple it's just easier to release content and base strategies around said abilities rather then thinking of all the variables included which make things harder to balanace.

However if customization is put into PVP with a point limit then if a skill combo is seen as to potent they can change the number variables or weaken said spell to balance it out, but the impact wouldn't be as big as in PVE because at least with PVP it's meant to be competitive and always balancing. Because of that it's easier for players to adapt into a new set of 5 BLU spells then it is to say completely level a new class/job because they gimped the one you are using.