I'm guessing they're going to go with some sort of corrupting effect to his gene seed as a side effect of whatever heresy brought him back.
But will the chapter actually care what the inquisition says about this? And if hes back at his post, what about the other absent primarchs? We've already got hints about Dorn and Vulkan still being around. Russ and Khan are also mia, but not confirmed dead.
But still, imagine the tactical benefit of finally being able to produce more of the imperiums greatest weapon. Not to mention Cypher may just steal some.
For some folks, this has got to be a nightmare. Gill has, potentially, control and loyalty of dozens or even hundreds of chapters of his own geneseed. Not to mention emperor knows how many guardsmen will flock to him.
And what will the other chapters think of all this? And the church? He's been afk for nine thousand years, but hes still a demi-god, so his word could radically change the order of things if hes not careful.
And Choas...the demon primarchs are going to be out for blood here. Every one of them will want his head as a prize.
I'm honestly hoping that the knowledge that hes back will lure some of the others out. Bring one out every few years till all the loyalists are back, and geat up for a grand reordering of things. After all, isn't the golden throne itself broken, and slowly failing.