I think that ESO has very little reason to add flying mounts. The PvE zones are highly compressed in terms of size and you have wayshrines everywhere for instant back and forth travel. And there is also a lot lost with flying mounts. You fly up, go to dest, then fly down. You no longer explore. And even WoW tried to get back that sense of size and exploration of the world by not having flying in their new zones last expansion.
All that being said, now if there was a zone dedicated to flying? Maybe something in obliviion or similar? That would be cool. But not in tamrial.