Author self-insert? Because it sounds very cheesy.
Chaos takes on Marvel
No he wouldn't. Doctor Strange is more powerful than both the Chaos Gods and the God Emperor. This is the guy whose job is to BTFO all the eldritch abominations of Marvel, who are also much more powerful than the Chaos Gods.
Cosmic superheroes are completely out of 40k's league and are capable of killing the entire setting pretty damn easily. For a comparison of scale, the Chaos Gods compared to the average enemy god of Strange are like the lesser daemons compared to the chaos gods themselves. They'd be a mook solved in single issue.
Ant-Man beats the snot out of all of them. Basically, kill yourself for thinking 40k is so high on the powerlevel list that it can tussle with capeshit. Nothing can rassle with capeshit except other capeshit.
Adding on to that...
Tzeentch is the fount of magic and literal creation. That puts him in the realm of The One Above All.
The material universe of Warhammer is not representative of the power of the Gods in their own realm, they are parasitic in nature. The very struggle against them feeds them through the paradox of their existence. The only plots that ever really had chances of stopping them were the Necron's old plan to close all gates to the warp and harvest all psychic "read, anyone able to do magic or psychic mind based powers" creatures into cattle of hopeless slaves, waiting to be devoured by the physical embodiments of entropy OR to kill off humanity right as they began feeding on them shortly after their births, starving them until they resort to the primordial warp stuffs they originated from, but seeing as humanity has constantly been threatened in the Marvel universe and each time the hero's save humanity from said threat... that aint logically happening.
Should a being of enough strength actually arise to challenge the "chaos gods" then they align and manipulate the mortals into achieving the downfall of their opponent.
Basically Khorne and Tzeentch, based on their concepts, are essentially unbeatable by the standard Marvel Universe, not without going "but this guy is supposed to be the strongest" and even then, once chaos is introduced, that character is only the strongest BECAUSE Tzeentch hasn't enacted a plan to overthrow them.
The only beings that threaten other Chaos Gods are each other, Tzeentch was once tag teamed by the 3 others when he crafted his rod of magic. But Khorne is also stated as able to kill the others, but doing so would fuck up the balance of the Warp entirely so he doesnt.
Bottom line, this is a dumb question. Chaos is designed to be inevitable, when you make it beatable it misses the whole point of chaos in the first place.
The One Above All isn't an author insert, he's literally -the- God. YHWH. Vishnu. All those omnipotent gods of humanity's mythology rolled into one that is the supreme being of marvel's canon. Nothing is more powerful than him besides the writer and editor. Although from a philosophical standpoint, one could say that he is the personification of all the writers, editors, and artists of marvel given his powers enable him to bitchslap anything he doesn't like on account of being all-powerful.
Could Marvel beat Powerman?
Galactus?
Dude concepts are completely beatable in Marvel. Marvel literally has a realm of concepts and they can totally be killed by anybody with the power. Death itself has been killed before so nobody could die. Marvel is the kind of place where you could kill gravity itself to make all atoms fly apart.
And no, the Chaos Gods are not remotely in the realm of the One Above All because the One Above All is omnipotent, by definition nothing is capable of coming infinitely close of his power because he is all-powerful, meaning nothing can rival him, or else he wouldn't be all powerful. The One Above All can end existence itself if he wanted, not only extinguishing the universes, but the very idea of it. He's YHWH. If he doesn't like it, it doesn't exist anymore.
>Says magic user can beat the being that is literally the fount of all magic.
>Tries to be serious
Nigga if Chaos is applied to 40k EVERY ONE of Dr. Strange's Spells, his very fucking spells, are Tzeentch's.
He literally cant defeat Tzeentch because his magic IS Tzeentch. He is awesome because Tzeentch lets him be.
Any "forms of magic" he makes are Tzeentch, any abilities he tries, are fucking Tzeentch.
His plans to seal the warp, ALL INVOLVE TRYING TO USE THE WARP TO DO IT.
Fuck I love Strange but seriously man... your missing the point.
By definition the One Above All is omnipotent, so yes. If the One Above All and Powerman met, they would have to be the same being, because there cannot be two omnipotent beings.