for me it's :
SC : Vulture
it just doesn't suit Raynor to ride a horse, he needs to use his ol' Vulture bike, same thing with Tychus or Nova.
WoW : Mechanostrider - Raptor
We still don't have any gnome or troll heroes (not to my knowledge atleast) but in case of their future additions I do hope we get to see some bipedal mounts such as Mechanostriders.
Barrel :
rather weird, but my first choice of hero was Li Li, and she was just too small so she looked kinda out of character riding a giant horse. so just give her a barrel to walk/run on. that was what came to my mind at that moment :3.
but anyway, what mounts do you like to see in the game?