Ok, I know what you guys are thinking; Gryphon. But in WOW, Gryphon is the signature mount of the Wildhammer Clan. I remembered the first time I logged into WOW and I found it strange that the human uses Gryphon as taxi. And the only iconic mount that had been stamped into the human race is the horse; armored horse, black horse, white horse, polka-dot horse. I find it strange that Blizzard hasn't come out something more unique for Human. The trolls have their vampiric bats, the mighty Orcs have subdued dragons and befriended wyverns, the gnomes have their flying machines, the Goblins have zeppelin. Heck, even the Blood elves have Dragonhawk while the Night Elves can ride their trusty Hippogryph.
I could see one day, if a great calamity arrive on Azeroth, the Chimeras would once again answer the call of Cenarius to be the steed of Night Elves and others who are exalted with the Cenarion Circle, so that would rule out Chimeras. The magic carpet seems to be something belongs to Dalaran. so not really the playable human faction while the frost dwarves ride their beautiful pristine eagles.
So I have an idea. Why not come out with a lore akin to how the Orcs manage to tame the Wyvern. I can see either bringing something new altogether such as Pegasus, to be introduced as the human iconic mount or maybe something already in game but just requiring a bit tweaking. I actually favour some sort of 2 headed Roc mount. It can be tied either to the Wastelanders or Stormgardes and somehow either of this faction made it possible for the human to include the mighty beast as part of their arsenal. Imagine a new banner for human consist of 2 headed rook, they would be the Humanity's main air assault unit in a war.
I also remember that pre-wc3 alpha, the Priestess Of The Moon was supposed to ride a giant Owl. I really would love to see the Sentinels or at least some sort of Elune-esque faction that flies on great Owls.