For example, you can only be Lv50 and under in ARR zones.
Lv60 and under for Heavensward, Lv70 and under for Stormblood.
This would fix the issue with S rank hunts dying quickly. SE would have to adjust the level of some quests mobs too.
There could be two layers of level sync, zone-sync, then another layer if you want to sync for a FATE that is too low for your level.