Hehe, Yeah you're the first one that notices it.
The Archmage will become a Defiler hero.
This because we've got some sort of a standard for classes and their factions.
2x light, 4x medium and 2x heavy armor per faction. And the Gryphonwing already had 2x light armor while the Defilers didn't.
Currently it looks like this: (Tank=Bold, Healer = Italic)
Gryphonwing: Rune Warrior, Ranger, Chanter, Stone Guard, Engineer, Redeemer, Sage, *Unknown*.
Defiler: Sorcerer, Zealot, Swordmaster, Death Knight, Assassin, Plaguebringer, Sister of Blood, *Unknown*.
Bloodbane: Berserker, Shaman, Witch Doctor, Summoner, Worg Master, Rogue, Enforcer, Survivalist