Given how ARR unlocks flying, I think it would make sense if subsequent expansions have both aether currents and ARR-style unlock as options.

ARR - complete 2.0 MSQ and unlock flying in all zones.
Subsequent expansions - do quests/aether currents to unlock flying sooner/individually per zone as you do MSQ or wait until you complete 3.0/4.0/5.0/etc MSQ to unlock flying for all associated zones.