Newcrons have that one Necron faction which controls a star map of the milky way - if they pluck out a star on the map, the star is destroyed in the real galaxy. Understandably a lot of people want this maguffin, and I'm not really sure how it hasn't already been used to win all of 40k. Then there's the Black Library, which probably still has the knowledge to make and unmake whole universes and gods and whatnot, if you ever remember to bring your library card to another dimension full of arsehole tricksters, more content with throwing chainsaws at you and shouting 'it was just a prank monkeigh' than letting you in.
The Tyranids sorta classify as a big world ending weapon as a species. I suppose a human victory would classify as a world ending event for all other species still remaining. Ultimate Ork victory may be survivable for a lot of the other factions, as they'd not put much effort into searching for hide and seek champions instead of krumping one another. The Tech Priests of Mars are sitting on some doosmday weapons (knowingly or unknowingly) in addition to the Void Dragon, but they're unlikely to ever touch or release most of them given the chance that said doomsday weapons may be sentient and full of mechanical rage aids heresy.
Full Chaos doosmday event may or may not be world ending, depending on interpretation of total warp victory. One still does wonder what the hell is going on in the other galaxies, if this much shit is going on in just the Milky Way.
Eldars win because Ynnead.
Tau win because superior tech in future and shit. Maybe.
Necrons win once they get their shit together and mobilize. They have spaceships that literally fire stars (like, completly and unbullshitingly stars).
Tyranids could win because they nomnom everything.
Chaos wins because everything is at war, life sucks and stuff.
Orks win if they finally get that huge Galaxy-spanning Waagh going.
Imperium... well, Imperium has many ways to go, actually. First is the God-Emperor getting his shit together in form of Star Child, Chaos God of Order or just in general Emperor coming back somehow. Another way could be Adeptus Mechanicus finally finding an uncorrupted full STC which would fix everyones problems forever. Or Primarches can return which might be less of an complete and utter victory, but would significantly make Imperium stronger... and then there is Ordo Chronos and whatever, those guys who asked Dark Eldar or someone for help, Imperial Guard going for standarized Krieg regiment production and drowning everyone in bodies, et cetera... and technically, Imperium does have nearly countless numbers. Not anywhere close to Orks or Tyranids, but there is retarded amount of people in Wh40k. IIRC, they calculated that they could technically give Tyranids an equal battle (implying conscription of all capable humans), but the problem is that Tyranids are not alone out there.
Everyone has a very good chance of winning (maybe except Tau but honestly I still don't get why they're a major race) without even plot armour and deus ex machina (well, maybe not literally because cogboys could pull the Emperor out of the machine). Some are just more likely than others, and IMO if we were ever to get into End Times for WH40k... everything whould happen at once, everyone would get their power mutiplied and resulting clusterfuck would shatter the Galaxy.
The way I see it, Eldar can win through Ynnead or if they manage to ally with some meatshields of champion attrition warfare capable of defending their colonies and allowing the Eldar to make the Eldar Empire great again. They're pretty much out of options on the latter though, as the last chance for such an alliance died with Eldrad. So they've only got Ynnead.
Dark Eldar don't really have a win condition, they just have persistence conditions, short of one of the Dark Eldar usurping Slaanesh or one of the Enshrouded completing their horrendous goals of modifying the DE/divorcing them from the materium and immaterium/other fucked up shit by DE standards.
Tau win condition would be developing their own webway tech, destroying the Hive Fleets and then bumrushing West to make a big Tau Empire. Then again they can also just sit tight. They don't really have any pressing objectives or things to defend beyond stuff being nommd by Tyranids, I guess their win condition is hegemony. I think it would be kinda funny if all these apocalypses happen and everyone dies, then the Tau just sort of move into an empty galaxy
Necron win condition varies because a lot of them have gone bonkers, but I suppose the one condition all of them agree on is separating the warp from the materium. If they are capable of doing that, no Chaos meddling, no psykers, no warp travel, no Eldar - pretty much the only people who could stand up to them then are the Tyranids and the Orks, and even then the Necrons possess the matter destroying weaponry capable of wearing them down through attrition.
C'Tan win condition is farming more souls and reuniting the shards and shit. We Grimdark Souls now, with Void Dragon still sleepy and the other shards all swooshing about doing whatever
Imperium win condition is bringing the Emperor back or fixing the Throne permanently / taking over the warp by ascending into a race of psykers more powerful than chaos (without being taken over by the warp) / making or taking over the webway gates / killing off all the other threats so they only have to worry about chaos / advancing the plot
Imperium has more options
I wonder if 40k is so grimdark that they even lost their potato STC. Life is not worth living without potato
Orks - orks always win, no matter what. If another faction takes dominance that just means better fightan, the only lose condition
is if there is no fightan ;(Tyranids win condition is presumably taking over the Milky Way and snuffing out the Astronomicon so that their hive mind can exist in peace, free from whatever is chasing them out of their home galaxy
Imperial Guard win condition is they hold the line and don't horribly die because of it
Adeptus Mechanicus win condition is they invent warpnet, and the sheer scale of shitposting the Imperium produces drives the Chaos Gods to suicide
SOB win condition is they run out of heretics to burninate in the thatched roof cottages so they can finally go back to their Emperor-grade husbandu pillows
Spehss Mahrin win condition is that they weaponize their blue steel rain modeling careers to smite down the enemies of the Emperor with sheer fabulosity
Chaos win condition is the Warp taking over the Milky Way, basically ensuring that their rule is concrete and nothing can challenge whatever their whims are
The stuff involving time is a bit messed up. Crypteks trying to delete time, Ordos Chronos possibly unmaking themselves (or existing out of time?), Warp beings floating upstream of time just as planned... Best not to think about it