-Flying while questing : Only have flight at max level.
-Flying to trivialize questing content at max level : make the zones not flyable until you have completed all the quests for that zone.
-WPvP : Besides Ashran, put WPvP objectives in the world that offer comparable rewards to instanced PvP content. (Towers to cap, areas to win and hold, choke points to over run etc, all rewarding honor, cp, gear and items/mounts). Increase the time to cast /mount a flying mount. Decrease the dmg needed to dismount a flying mounted player target. Make the announcement of PvP logistical targets being attacked appear in trade chat on Ashran.
-Size of the world : Make explorer a requirement to unlock flying, nerf the speed of flying, lower the sky box.
-Max level daily/weekly/quest area trivialization : Make those areas unflyable (like IoT, TI, MF etc).
-Making flying relevant : Add content that highlights and uses flying.