Whenever (if at all) Vulkan drops, he better be Guilliman sized IMO, yeah sure G-man has grown quite a bit but it'd feel wrong if he towered over Vulkan.
Just remember that Guilliman is carrying his own life-support system as well in his armour, he is much bigger than a standard Primarch and I suspect any other primarchs coming out would indeed be infantry. Heck he's the size of a Daemon Prince (if not bigger) and they're monsters...
Probably a moot point however since there's probably 5 or 6 primarchs coming out before Vulkan anyways
To me Vulkan is a prime candidate to return and I'd love to see him. What date is Abaddon due?
Abaddon is due 9th March I believe, I presume this is pre order date as that is what GW seem to use now rather than release date.