What would We call it,

The Flesh Dilemma?

Pardon me for saying so, but this seems to be off. Way off in many respects. I certainly agree upon the idea that certain legions would be hostile or loose cannons in the wake of the Great Crusade's total success, perhaps to the point of possible rebellion, but I don't think it would turn out in this way. Even ignoring the personalities involved, the Emperor tended to have things set up in the long run to cover up these issues. We saw the Thunder Warriors destroyed at the end of the Terran Wars because of their violent, unpredictable nature and others wiped out after they had no further use, but could not be acclimatized to a time of peace.

Even ignoring the idea of the Emperor having contingency plans against this, the legions were not so wildly rogue that they were not ready for it. A Thousand Sons and a few other books pointed out how certain primarchs were actually preparing their astartes for a time when they were not needed in such widespread numbers, or focusing their attention towards other arts. In particular, Ahriman was noted to be considering taking up wine tasting and a few other marines had found other outlets. However, this was only to the point where they would not be in such widespread use, the Imperium would likely always need the astartes in some form.

Some hostile xenos races escaped purges to threaten the Imperium time and again, often being enough of a threat that the Imperial Army alone was not enough to fully combat them. This has been seen both in M31 and M41 alike, and with the Emperor's intended end goal of protecting humanity, it seems unlikely even he would be blind to such an issue. The orks, as some pointed out, were still a major open threat, and likely one which could never be fully purged. The Craftworld Eldar were always a looming dagger over certain worlds, the Dark Eldar still existed within the webway and many later threats had yet to fully arise. Even at the time of Galaxy in Flames, it was noted that it was not uncommon for the legions to be going back over long conquered Imperial territory to deal with any cracks which might have arise.

Still, the idea does have some merit, but I personally do question the choices of legions involved.

Both are obvious choices in this matter and i'd personally agree they were the most likely to go rogue. They were weapons of fear and annihilation, intended to either completely destroy an enemy via the chainaxe or inspire such a degree of terror that none would break the Imperium's law. The long issue I would question would have been if the World Eaters would have gradually destroyed themselves or possibly have phased out their most extreme aspects. Angron was dying thanks to the Nails after all, and it is possible that his legion would have followed in due time or that they even might have reversed the same policy of implanting them in legionaries that he had enforced.

Now, this one I do have to question. On the one hand, the Death Guard might have turned given Mortarion's view towards tyrants and hatred towards psykers. That said, Perturabo might not have followed him. He was certain belligerent and violent in his approach to war, but Angel Exterminatus did confirm that he was more unhappy with the role he was performing that resenting the Emperor himself. Whether or not you agree with that novel's version of the character, he was a siege master who wanted to ultimately be a builder or construct a new version of Olympia on par with Ultramar. Peace would have given him that chance, and the opportunity for his legion to be so heavily divided. He might have still turned given his treatment and lack of any true glorification, but given it took Olympia's rebellion to finally push him over the edge, i'd argue if that hadn't happened he would have possibly stayed loyal.

Perhaps, but they seemed more than happy to spend their time wiping out ork strongholds for fun. They were less war happy than thrill happy at times, and I could see them taking to other roles personally. Perhaps assisting explorator fleets in investigating new worlds, continuing to bolster the librarium they had helped found, or even another role. The Imperium webway did, after all, likely still need guardians. Who better to do it alongside the Thousand Sons than a legion who venerated psykers and focused upon speed above all else. Well, assuming they wouldn't need to take the Thousand Sons' natural place in guarding it of course.

... Pardon me, is this a troll post? Apologies if this is unintentional, but you managed to get everything here wrong, even the legion's name. First of all, the Iron Hands' ideology wasn't taking themselves further and further away from humanity to the point of rebellion. "Flesh is Weak" didn't refer to humanity or flesh itself, but of human failings and their intolerance for personal weaknesses. Their ideology was focused more upon perfection and self-improvement through any means. The emphasis upon bionic enhancement above all else would only come following Ferrus' death, after it was realised they would need to become monsters to fight even worse monster. Prior to that, their primarch wanted to actually put an end to this trend at the Great Crusade's conclusion, when it wound no longer be needed.

Also, "fits like a kindergardener"? Are you certain you have the right primarch in mind there? The only moment in which that happened was after the Heresy was revealed. You know, when a bunch of astartes turned traitor, and he was so insulted he wanted their heads put on pikes. Bare in mind this was the guy who not only first refused any option to turn traitor, but was loyal to the point that even clones of the guy would turn violent upon any attempt to coerce them into treachery.

Much like the Iron Warriors, i'd personally have to argue otherwise. Along with revering the Emperor as a god, one of the reasons for their humiliation was their insanely slow progress. They were building new worlds, but at a much slower speed than the Ultramarines, causing massive delays in the Crusade. With its end, it would be an excuse for them to go back to what they had been enjoying doing before being ordered to stop. Plus, what better way to get back at Guilliman than beat him at his own game.

This is the same legion trusted to take down other legions upon going rogue. Much of the traits you describe were a front to a fair point and they were noted to be surprisingly highly disciplined despite their barbarian origins. Even if they couldn't slake their rage against orks, they would find another way sooner or later. You might have a point about the wulfen mutation though, but it depends how well they could keep that under control or who knew about it.

Varies. It's possible but unlikely that this would happen. He certainly saw himself as first among their number, but it never showed any signs of reaching the point of arrogance or resentment you describe. After all, it was noted he just took Horus' elevation in his stride and just continued his work as per usual. Mortarion or possibly Fulgrim might have reacted in the way you describe, but I personally can't see this happening given the facts established thus far.

Most of the loyalists I actually agree with you upon, though I would argue that the Blood Angels would run the same risk as the Space Wolves thanks to their own flaw. That and the Thousand Sons might not have been about given their slowly dwindling numbers prior to Magnus' deal with the devil.

If the Great Crusade was sucessful I think the Emperor would have gone back to make the rounds again, incrementally increasing the requirements of compliance in the attempt to delete all forms of deviant behavior, tightening his stranglehold on the galaxy. The Cult Mechanicus being the only religion endorsed by the Emperor is quite telling, I think society would have gone more in that direction, seeing as he would have eventually realized that the Chaos Gods are fed by our most base emotions that in order to truly defeat them that he'd have to strip humanity of their humanity, reforging mankind into humanoid drones whose only emotion is trust in the Emperor, perfect little cogs in his galactic machine.

So basically I think the utopia he offered would just be another form of hell.

I stopped at 'Iron Fists'.

Thanks for pointing that out, fixed the legion name.

well if it came down to attrition Id have to go with nids, every ork slain and nid slain would be used as biomass to create more nids xD

The way I interpreted the novella in Primarchs was, they showed near open disdain to mortals because of their weaknesses. Only after the humans made a courageous imperial style last stand kamikaze that Iron Hands seem to accept them. Their mentality seems closer to that of mechanicum than humans. They also see their implants as straight up improvements upon human parts, and even punish one of the legionnaires for getting rid of his arm to fight eldars. In the same novel Ferrus is in a constant process of rage, and it is talked as a great accomplishment when he can keep is anger in check.

His anger imo would easy to be manipulated by a primarch. Traitors could pull a "Magnus like event" and put Ferrus in a situation where he charges head in and wouldn't be able to go back. Fulgrim's argument wasn't very subtle to begin with.

Fixed the legion name, thanks for pointing that out.

Being very slow was part of it yes, but the main problem was nearly all the world they brought to compliance were treating the Emperor as a god. So all those words were against or borderline against the imperial creed which was unacceptable for the Emperor. So it was basically defeating the purpose of bringing those worlds into the imperium.

if you heard about "The First Battle of Fang"
you will know that Space Wolves where close to curing the wulfen (till magnus destroying their labs)
if that is the case i don't think that they would ever betray the Emperor (even with wulfen it would be somewhat impossible)

Actually no, Killbo is correct. The Emperor ignored it for the better part of 100 years UNTIL it started to impede the progress of conquest. Macragge for example had a Catholic church on it, now you can infer that the Emperor was going to purge it later(He planned to do the same to Psykers based off of Daemonology.) but for the moment he was giving them a pass.

Thus, Lorgars confusion about his fathers sudden hostility towards faith.