system
Given the following post, respond as if you're commenting on Reddit:
human
[WK40k/Doctor Who] The Doctor arrives in the 41st Century and decides to get everyone to stop fighting. How would he go about it?

The 41st century is pretty grimdark, but Doctor Who is a space magician who travels in space and time. Assuming bringing some sort of peace to the galaxy is possible, only Doctor Who has a good chance. It would probably take him up to 90 minutes though, 45 mins for an entire galaxy of Tryannids, Orks and Necrons is pushing it.

"He can't" isn't an acceptable answer in this question, as Doctor Who is really good at stuff like this. So how could he fix things up with as little loss of life as possible?
gpt
* Sonic Screwdriver the Golden Throne into functioning (a good episode would show him delving into its labyrinthine guts to find something that needs fixing, dodging seraphim and servitors on the way). The Emperor awakens, and psychically speaks to the trillions of subjects who -against his explicit wishes all those centuries ago- now revere him as a god. He brings down his own church, instead asking for Mankind to stop fighting itself, stop being so insane and cruel, and to actually progress one iota for the first time in millennia. He outlines the scientific method to the tech-priests, teaching them to explore ideas with rational minds rather than by rote adherence to "rituals" so old their true purpose has been forgotten. This done, he either unifies mankind and causes a technological revolution, or demands that he be allowed to die, depending on whether the Star Child theory is correct, complete with an emotional scene where he asks The Doctor for his help in dying.

* The Doctor gets the attention of the Orks by flying the TARDIS straight through every ship in a a Waaaagh, making him Da Biggest Boss (while The Doctor traditionally hates to kill, he \*has\* blown up entire fleets of Cybermen just to make a point). He points out to his rapt audience of survivors that they're the killiest species in da oonivers, and that fighting the others is frankly beneath them. He tells them of a star system filled with their greenskinned brethren for thousands of years, Orks who have lived on Hellish worlds that have made them tough, and strong, and cunning, and smart, and who sent him to tell them: come and have a go if you think you're hard enough. By sheer coincidence, the route to this system means travelling directly through the areas of most concentrated Ork population, ensuring the maximum possible number will be involved in this new Waaaagh. Once arrived, the Orks will enjoy millennia of fighting amongst themselves, well out of the way of the other species of the galaxy.

* The Doctor walks directly into the path of a Hive Tyrant, which immediately recognises him as Something Else by his psychic signature. He requests and is granted an audience with the Hive Mind itself. If the Tyranids are running from some bigger bad, as has been postulated, The Doctor gives them the means to defend themselves, on the proviso that they never use it against the inhabitants of this galaxy (and he'll be watching!). If they really are just ravening beasts, he performs some Deus Ex Machina that turns the connection into a 2-way psychic transfer, allowing him to manipulate the very essence of the Hive Mind and mellow it out, allowing for a possibility of a Tyranid species that can exist in balance with its surroundings, rather than just consuming everything.

* Rewrites the base programming at the core of a Tomb World, propagates this update to all others. The new Necrons gain a new perspective on their aeons of bitter warmongering, and dedicate their long, long lives to bringing their brethren out of stasis and living alongside all the other species of the galaxy in relative peace, despite their misgivings about being surrounded by the now technologically advanced animals that were created by the Old Ones specifically to destroy them. Eventually, Necron mercenaries become a common sight among the stars.

* The Eldar are hot blooded jerks with too strong a psychic link to do anything with- last time they got powerful they spawned a Chaos god. They're also too proud to consider genetic manipulation that would temper this aspect of themselves or make them more level-headed. Making them accept they they need to change would be extremely OOC, but if anyone can do it, it's The Doctor. Alternatively, The Doctor could simply force the genetic manipulation on them, either covertly or with brazen gloating, as the Eldar have been known to utilise artificial wombs and multi-stage biological impregnation that could easily be tampered with- one dose of Doctor brand gene therapy on an unsuspecting father could introduce a dominant allele that would do whatever needs to be done. A few (dozen) generations later, and the Eldar are a different species altogether. Inhumane, borderline evil, but the only choices are to change or be eliminated in a galaxy of peace, so it's the lesser of the two evils.

* The rejuvenated Emperor shuts the Eye Of Terror, limiting the influence of the Chaos gods severely. The galaxy will never be free of them, not while sentient beings exist, but they can be curtailed, at least. If The Emperor dies and kicks their collective asses as suspected, this one is moot.

* The Doctor uses DNA and chemical analysis to prove that the Tau are being manipulated by the Aun caste, and leads them in a revolution. After centuries of development, the formerly warlike and aggressive Tau castes have enough perspective to see the benefit of collaboration, in the same way humans no longer attack one another on sight like cavemen or earlier hominids. Without their drive for galactic dominance, the Tau's expansion stalls, and while they -along with every other species- still have plans to dominate the galaxy, it's on the backburner for now. A massive, unimaginable cultural paradigm shift just happened, maybe it's time the Tau invented comedy, or music, or something else not strictly related to war or the support thereof.

Any more for any more?