Bloodmyst and Azuremyst are pretty much ready to go already. The only reason they don’t have flight is out of a sense of fairness.
Eversong/Ghostlands unfortunately are disasters in terms of what needs to be done to support flight. Silvermoon is the biggest cardboard castle in the game right now and it falls apart badly in the air. To make things worse, down south in the Ghostlands, Eastern Plaguelands is entirely missing if you’re high enough to see past the southern border mountains, and in place of shores on the east and west there’s sheer cliffs. Additionally, there’s the whole mess of how Deatholme in Ghostlands occupies the same space as Stratholme in Eastern Plaguelands.
Argus, the Maw, and Korthia would probably be just a switch flip.
So out of those the Blood Elf zones are the only ones that’d take work, and they’d take a lot of it. Minimally, you’re looking at rebuilding Silvermoon from scratch and doing a bunch of terrain surgery in Ghostlands to make things make sense.
Interestingly, it appears that during Cataclysm’s development they may have been considering making Eversong/Ghostlands flyable, because in that expansion they removed the giant troll gates of Tor’Watha in Eversong which had nothing but dirt behind them and would’ve looked silly from the air. They evidently scrapped those plans though, probably due to lack of time and resources.