Vulkan: An even bigger hellhound with even bigger flamers.
Roboute Guilliman: Some kind of Mobile command vehicle, maybe something like that huge land fortress Leviathan thing.
Sanguinius: ???
Corvus Corax: I would imagine some kind of light tank, a skirmisher or something. Or perhaps a stealth tank?
Lion El'jonson: ????
Jaghatai Khan: Hmmm, Some kind of fast-moving tank destroyer?
Ferrus Manus: ???