Rich Lambert said it'd never happen because players with their current mounts are already getting into places they were never intended to enter. So that's a hard no.
Won't ever happen
1.Reworking ALL the existing map to take into account flying mounts will take way more ressources than it will give back
2.In those unaccessible areas you will not see anything more than you can now
3.Having to make such area will cause a massive increase of disc space for quasi nothing, causing disc space issue on consoles and performance drop both consoles/PC
4.PVP/Cyrodiil, what's the point of walls if you can fly above them, it will be called out as unfair advantage
5.The game never had verticality in mind even for basic movement so flying mount will be nothing but a huge mess, only chapter featuring this is the Elsweyr chapter with the grapplin hook, its already clunky and never used anywhere else (what I call a chapter gimmick mechanic, somethign unusable outside the area the DLC was attached to)