I'm still kinda of the opinion that Guilliman should be the one to take Molech, rather than the Lion. It does lessen the Lion's power, but it is the Guilliman Heresy. If anyone's to gain powers rivaling the Emperor's, it should be Guilliman. It's looking it might just be one of those thoughts I've had and cemented into my idea of our canon without ever posting it, but I think that Guilliman going to Molech while the Lion is besieging Terra to be the best course.
. . . Maybe we can all have our cake and eat it too. The Lion attacks Terra in the biggest battle fought in the Heresy up to that point. Worse still, the Emperor's greatest project comes under direct threat. The machinations of the loyalists keep the Lion's greatest weapon, the Fists, out of play. His hidden blade beneath the palace, the Scars, begins to have doubts. The Chaos-engorged Lion does succeed in killing Fulgrim, but is unable to stand against an Emperor that has finally chosen to take part. The Siege ends, and the traitors scatter before reuniting under Guilliman's banner.
Guilliman, at this time, is on Molech. Already a world of his own, bastioned as it was by his own sons and that of Sanguinius, no battle is necessary. Well, there is that perpetual guardian, but she wouldn't stop Guilliman any more than she could Horus. What happens in there, happens. Oh! Okay, I know none of this was posted, but I remember now more of my thoughts on this. The Chaos Gods have two pawns in this Heresy, with the Lion proving the greater simply because he's willing to go on the offensive. But while the Lion was on his offensive, the Guilliman was planning for the future. He knew the Lion would not succeed, and that his failure would bring the Emperor fully into the war. Just as Horus learned of Molech, so too does Guilliman.
Guilliman enters the portal, and proves his worth to the Gods. This has two direct effects. One, Guilliman walks again on Molech as an ascended being. Two, mirroring canon Horus, the Lion feels the Gods abandon him. The Lion falls, yet another Warmaster remains.
Now, that's basically been my thoughts on the matter, from way back when we were still updating regularly. I guess since then I internalized it without realizing I was the only one who even knew of it. But here's where we can both get what we want. Molech is a portal to the Gods. It's a direct link to their realm. The Emperor used it to steal the knowledge necessary to create the Primarchs and begin his greatest work. Horus and Guilliman used it to gain the power necessary to rival Him.
But it's still basically a hole from reality to the very domains of the Gods. And that hole can be torn wide open. Molech can become a Daemon World in realspace, an endless supply of daemonic beings rampaging forth to bury the globe. And with his newfound power, why would Guilliman stop there? The Emperor is coming. Bind Him behind a daemonic border. And so Guilliman and his more gifted sons travel from world to world, enacting rituals that tear great holes into the Immaterium. Each damned world linking to the other across space, giving rise to the immense Ruinstorm that divides the Emperor and his still-loyal forces from the Five Hundred Thousand Worlds of Ultramar (trademark pending).
And when the Emperor's forces are thrown adrift by the Ruinstorm, His retribution stayed before it could begin, what choice would he have but to locate the lynchpin world and end the Ruinstorm? And so the Emperor, his Custodes and the Silent Sisterhood leading the most elite of the Emperor's forces, lands upon the daemon-infested planet of Molech. In a battle somehow reminiscent of the canon Heresy's War in the Webway, the Emperor fights his way to the portal his wayward son had torn open and after a terrible battle even for one such as He, finally seals it shut.
With all the other tears linked to Molech, feeding off of it, empowered by it, the loss of it would cause the scars to sputter, scab and heal over. The flow of daemonic forces cease, the great warpstorm fading to mere turbulence. A commendable defense still, but no longer insurmountable. The Scouring of Ultramar proceeds.
But wouldn't Molech just be the perfect place for the Emperor to fight Drach'nyen?