Heirloom items

[Enkidoh]

There is already EXP boosting items in the game that level sync their stats according to your level - most are earrings given as preorder bonuses for preordering the latest expansion (the most recent is Menphina's Earring that although level 1 by default, level syncs continuously up to a maximum of level 80), but there are some rings that boost EXP as well (these don't level sync though but only give EXP boosts to a certain level - the Brand New Ring from completing the challenges at the Hall of the Novice at level 15 is an example).

Also levelling alt classes and Jobs is already given a default boost due to every class/Job that is lower than your highest one gets a boost to EXP gain until they reach that level (like, you have one Job at 70, every other class will then get a boost to the EXP they gain in battle up to level 70. On top of that there is also Rested EXP gain from spending time in Sancturaries (which you should be familiar with from WoW), Free Company (FC) 'Company Actions' that give an EXP boost to their members, consumable items that give a similar temporary EXP boost to the FC actions, and the 'Road to 80' boost for starting or switching servers to a new server or server that is considered low population. Hence we have plenty of ways of boosting EXP gained already.

So there is already things like this in the game.