How the Hell could that happen with the 'nids, the orks, the necrons and, well, the current dominion of the Great Enemy upon the galaxy?

I rather think that the disaster will occur when they will somehow try to resurrect the Emperor.
I also believe that Malice is a shred of the Emperor's consciousness turned mad after the Horus Heresy, and so that one day the fifth Chaos god will finally be complete and start a new reign of (silent) terror at the antithesis of what the Lord of mankind had accomplished during the time of the Great Crusade (that is of course pure speculation, but 'would be fun though ).

As I have stated many, many times, me, and several of my kinsman, like things exactly the way they are.

To be fair, you are also insane.

It would also beg the question if who the Emperor is within his meat shell is even relevant, or if it would be soley a manifestation of the Imperium's faith, as the Emperor is more a symbol than anything. Would this newborn god be a spirit of wrath, consuming all impurities within its realm, or perhaps the pure themselves? Would it strengthen the Ecclesiastiarcy or destroy it? Hell, would it turn upon its own Empire for what it has become?

Such questions...

Hey, I'm not the one who wants to see the Fall happen AGAIN.

Much thinking
Wow

Since negative emotions seem to dominate all to others within the Immaterium, I would say that this newborn god will be another manifestation of destruction; a sort of Antichrist seeking to divide mankind rather than to unify it, to bring anarchy in the galaxy rather than order.

All beholden the Chaos God of Fanaticism, Genocidal Purity, and Wrathful Divinity. The most holy Terra shall explode in a ball of such pure energy with the cleansing fires of a thousand suns that no mortal can hope to withstand in its brilliant judgement. As it expands into the material realm countless worlds will be consumed by its holy light, leaving only daemons of insurmountable faith in pursuit of all things heretical. The doubts, fears, and criticisms of its followers shall burn through the consciousness of all it touches, purifying itself in a great supernova of devotion. The only way to stem the tide of its consumption will be to open a string of black holes to devour the outflux of raw energy until the material, immaterial, and void clash in a cataclysmic tide to give birth to a new universe.

Or not.