Deep Dungeon: Several suggestions for improvements.

[Fareeha]

For Example: Take Flash for Paladins and add 3 different customisations for it.
Gem A: Increase the potency of your blind, but the blind can no longer be refreshed past its original 12 second duration. - Purpose: Short bursts of serious damage mittigation.
Gem B: Add a paralysis effect. - Purpose: Similar to Gem A, but focused on prolonged fights. Especially good versus casters.
Gem C: Remove any debuff caused by Flash, increase its manacost, but allow it to deal damage. - Purpose: For those who prefer straightforward damage.
Only 1 Legendary Gem can be equipped at any given time (for each modifiable skill)
Examples for other jobs: You can add a pin (bind) effect to Death Blossom (NIN), allow Empyreal Arrow (BRD) to apply both dots, add a ricochet effect to the Rook Autoturret (MCH), add a second strike effect to Rockbreaker (MNK) (after 1 second, the ability goes off again, without cost or cooldown), upgrade Low Blow (DRK) to not only stun, but also kick the target halfway accross the room.