One thing EQ1 did really well for level capped characters was offer an "Alternate Advancement" (AA). This system enabled players to continue to acquire experience, but to apply it to AA points. A player then allocated AA points to specialty (AA) abilities to further specialize their character. I really missed this in other MMORPGs.
The experience required for each AA point was significant, requiring more of a grind. Choosing not to grind AA's did not make a character ineffective; it just didnt have a slight buff to certain abilities.
The enhancements, whether for dps, tank, healing or support, were only minor improvements, but did enable toons to be somewhat more unique with individualized areas/abilities of improved strength. For example, an archer (ranger) could improve bow skills by earning a slight passive increase to bow speed. A healer might improve their rez, such as a slightly lower cd for battle rezzes or returning a player with slightly more health and mana; A tank might gain a passive slightly improved mitigation; etc...
Having some kind of AA system would give additional reason for completing undone quests post level cap, help players feel that they are still improving, albeit in more of a grind, as they wait for lfg to pop or in between raids. It would also be kind of cool to go into group play knowing your toon has a unique slightly buffed ability (that you worked hard for).
Anyway, would love to see something like this. In some ways its merely and extension of soul trees, but only slight buffs to present abilities and much longer to grind out.