So, if we look back a few expansions, The Lich King dropped Invincible's Reins, Madness of Deathwing dropped Life-Binder's Handmaiden.
Question is, what do you think the mount will be from Heroic Garrosh? And what do you wish for it to be?
We do know that the achivement for killing Garrosh on normal difficulty prior to next tier will reward a wolf, so I think it's safe to say it's not a wolf from heroic mode. I would like to see something new, as in not a recolored/new armor of one of the old ones, would be nice to see a new unique mount, much as Invincible or Mimiron's Head was when they were released.
Edit1: It's official that he will have a 100% drop rate mount, it's on the frontpage confirmed by Blizzard.
Edit2: It's now also official that the mount will be a Kor'kron Juggernaut. In the tooltip it says the following "Dropped by Garrosh Hellscream - Siege of Orgrimmar." - http://ptr.wowdb.com/items/104253-korkron-juggernaut