One metal schlong to rule them all. Also, the Primarchs sure as hell shouldn't show up to the planet without breaking the background over EC's knee. If the Primarchs show up in battle, everyone dies.
Already done I personally don't want to see a primarch that much, that just means that they will obliterate the other factions also because primarchs are like giant hairy bald cockroaches, if does something interesting, the rest will come crawling as well, well all the ones that arn't dead or sitting on a porta potty, or sitty on a warp potty thinking about the meaning of life *Cough* Lorgar *Cough*
meditates, on the true purpose of Chaos and Gods in general, the nature of the warp, the battles to come, etc
I think the deamon princes will send there champions forth and from them will come are new chaos lord for Arkhona.