Okay, Just a heads up, I may be ad-libbing, but that's because I do not remember what actually specifically happened in Infinity War. Please forgive me!
(Deadpool was standing semi-heroically on a tall cliffside, staring up into the black, night sky)
Cable: (walks up) Uh, Deadpool, what're you doing?
Deapool: (looks at Cable seriously) Thanos is coming.
Cable: Thanos?
Deadpool: Big purple, raisin-headed alien. Stupidly powerful 'cause the writers adore sooooo much. Total ripoff of Darkseid. And he intends to steal the six Infinity Stones to erase half the universe from existence 'cause he's obsessively convinced of overpopulation.
Cable: Then why not just use the Stones to double or triple the universe's resources?
Deadpool: No one knows, Cable. No one knows.
Cable: Well, why worry? The Avengers can stop him.
Deadpool: Not quite. See, I snuck into the Sanctum Sanctorum and used the Time Stone to look at over 14 million futures. It was only on Future 14,000,605 that I saw one where... er... okay, I can't lie, it's not a "victory" by a long shot. Thanos still wins, gets exactly what he wants, we just wait five years and undo it and then are just expected to nonchalantly pretend nothing happened. What a rip-off, I coulda done that on Future 1. Seriously, what was the difference!?
Cable: Are you serious!? That's not a victory, that's just stealing the trophy!
Deadpool: Exactly. Still, it's nothing compared to the comic book version of events.
Cable: Then I guess we will have no choice-
Deadpool: Pffft! Are you kidding me!? I wouldn't stop there! That was a stupid future! I kept going, and wouldn't you know it, literally every Future after 14,000,605 was a way better one! Ones where Thanos definitively lost!
Cable: Oh. Well, then, what shall we do?
Deadpool: Well, Future 14,000,606 involves messing with space and time, soooo, we'll use that as a last resort. I'll tell you everything else I saw.
Future 14,000,607
Upon the Asgardian ship, Thanos is, quite simply, murdering everyone 'cause he's a jerk. Bruce and Thor have been defeated but Heimdall is readying a teleportation for Bruce to save his life, when...
Deadpool: Psst, hey Heimdall?
Heimdall: Huh? Who are you?
Deadpool: Never mind that. I just came to suggest, why use your last breath to teleport Bruce to Earth when you could teleport Thanos into the vacuum of space?
Heimdall: ...Oh, yeah, why didn't I think of that? (teleports Thanos away)
Thanos: (appears in the vacuum of space, far away from any planets or stars) Gak! (begans choking)
Future 14,000,608
Thanos: Destiny, it always comes. And now it's here. Or, you could say, I am here.
Deadpool: Um yeah, gonna stop right there, Raisin-head. Just thought I'd introduce you to a friend.
Carol: (appears, and fluffs her hair) 'Sup.
Thanos: Oh, phooey.
(Carol singlehandedly annihilates Thanos and his entire army)
Thor: Deadpool, I think I'm in love.
Deadpool: You and me both, drape boy, you and me both. (looks at camera) So, just to clarify, the next three futures are also me bringing in Carol against Thanos, on the Reality Stone planet, Titan, and in Wakanda, soooo, gonna save us some time on that, okay?
Future 14,000,611
(Thanos has thrown Gamora off a cliff... until Deadpool swoops in with a grappling hook and saves her)
Thanos: What the?
Gamora: Huh? What just happened?
Deadpool: 'Sup. I'm Deadpool. Okay, so we're gonna get you and your sister as far from your dad as humanly possible. You two are the only creatures in the universe he cares about, so if he can't kill either of you, he can't get the Soul Stone.
Gamora: Oh, yeah, good call.
Deadpool: Y'know, if things don't work out between you and that Quill guy... you know he literally doomed the universe over you in one timeline, right?
Gamora: (gasps, before sounding touched) He doomed the entire universe for me?
Deadpool: Oh, come on!
Future 14,000,612
(everyone is confronting Thanos on Titan)
Thanos: I take it Maw is dead then. At least he accomplished his-
Strange: Cloak of Levitation, (it pulls on Thanos's arm) Sling Ring, (a portal falls and Thanos, sans his arm, falls through it) and bam! (portal closes and chops off Thanos's arm)
Deadpool: Awesome! See how much easier that was!?
Strange: Yes, yes, thank for the suggestion.
Deadpool: What can I say?
Strange: Yeah, yeah, I get it, I gave up too soon.
Future 14,000,613
Thanos: I take it Maw is dead then. At least he accomplished his-
Deadpool: Surprise, Motherblanker! Time Stone!
Thanos: What the- (in a flash of light, the two are in a dark void)
Deadpool: Welcome to the end of time, Raisin-face. Enjoy your stay. Oh, and uh, (shows the other four Stones off in his hand) please leave all valuables with your valet on your way out. (returns to the present)
Thanos: Oh, phooey.
(in the present)
Deadpool: See, that, Doc? When you've got a gemstone that warps reality, you use it properly.
Strange: Yeah, yeah, I get it, I gave up too soon.
Future 14,000,614
Tony: We'll go to Titan. Maybe we can find something there to-
Deadpool: Hey! It's me, Deadpool here. I just had a suggestion. How about you don't go to Titan because nothing is waiting for you there, and instead head to Wakanda, where all your other superhero friends are waiting?
Tony: What? No way. Strange knows how to-
Deadpool: Okay, yeah, spoiler alert, the future he saw involved just letting Thanos win, making whoever was left, which, BT Dubs, excludes you (points at Star-Lord,) you, (points at Spider-Man), you, (points at Mantis) you, (points at Drax) and also Strange himself, wait five years, and then just breaking space and time to just undo it.
Everyone but Strange: WHAT!?
Strange: Uhhhh, I, I just-
Deadpool: Doc, do us a favor and look into that stone thirty more times.
Strange: (looks confused, but does so) ...Okay, so, I gave up too soon. Yes, let's go to Wakanda, shall we?
Tony: I don't know, Cap and Bucky are gonna be there...
Deadpool: Okay, Tony, for the love of Pete! Thanos is out to destroy all of existence! Who CARES about your mom and dad right now! For the sake of the universe, can you please JUST LET IT GO AND GET ON WITH YOUR LIFE!
Tony: ...Fine, let's go.
(in Wakanda)
T'Challa: You are in Wakanda now. Thanos will have nothing but dust, and blood.
Proxima Midnight: We... (looks on to see the shear size of the good guys' army) ...are definitely gonna die.
Wong: (suddenly appears) Hold it, everyone. Mirror Dimension! (the entirety of Thanos's army disappears)
Deadpool: Again, I say, you got reality-warping powers, Doc, you should use 'em properly.
Strange: I get it! I gave up too soon!
Future 14,000,615
Vision: You must destroy me. We cannot allow Thanos to steal the Mind Stone.
Deadpool: Oooooorrrr, you could try using that stupid Stone. You know that thing can control minds, right? Or at least, try fighting a little more. You're like, super OP even without your Danny Phantom powers.
Vision: Hmmm... Okay, let's do it.
(later)
Corvus Glaive: Give me that Mind Sto-
Vision: (imitating Obi-Wan with the Mind Stone glowing) This is not the stone you're looking for.
Corvus Glaive: (in a trance) This is not the stone I'm looking for.
Vision: You should kill yourself right now.
Corvus Glaive: I should kill myself right now. (stabs himself in the chest)
Thanos: (Arrives) Hey, what do you think you're-
Vision: This is not the stone you're looking for!
Thanos: Your Jedi mind tricks will... uhhhhh...
Vision: You should annihilate your entire army and then kill yourself.
Thanos: I should annihilate my entire army and then kill myself. (runs off and does just that)
Deadpool: As I said, you have reality-warping powers, you should use them.
Strange: I get it, I gave up too soon!
Future 14,000,616
(in Wakanda)
Thanos: At last, I have them all!
Deadpool: No, there is another...
Thanos: Huh?
Deadpool: (whips out violet gemstone) And this one is kinda the best: the Continuity Stone!
Thanos: No! I-I thought that was just a myth!
Deadpool: You kiddin'? Eat your heart out, Raisin-head. This is what happens to the one guy whose even more adored by the writers than you! And now, I wish Thanos was never born!
Thanos: (starts fading out of existence) No, no!
Deadpool: And with that, your power of reality-warping does not compare to my power of... FOURTH-WALL BREAKAGE!
Thanos: NOOOOO! (disappears)
Deadpool: And now, I wish Quicksilver didn't die, the Avengers were still together, Han and Luke were still alive, Kristen Bell became a model instead of an actress so that Good Place was never made, I wish for Eric Kripke to get hit by a car before he can pitch Supernatural...
Future 14,000,617
(Thor arrives in Wakanda and throws his StormBreaker axe at Thanos... until Deadpool suddenly swoops in, grabs the axe, and charges Thanos with it)
Deadpool: Yippee ki ya, motherblanker! (beheads Thanos as quickly as possible) That's right! Deadpool is worthy!
And now for the very best of the Futures: lucky number 14,000,618!
(in Wakanda)
Thanos: Come on, my Children. We will burn this entire country to the ground if we must!
Deadpool: Um, yeah, about that, I saw another future that had all these portals and stuff, so I figured, why not do it here? But even better!
Thanos: Huh?
Deadpool: (whips out the Continuity Stone again) Say hello to my little friend!
(the stone opens several portals, but they connect to other dimensions)
Deadpool: Meet the superheroes who are just naturally better... DC!
(Out of one portal, Superman, Batman, Wonder Woman, the Flash, Hawkgirl, and Martian Manhunter appeared. Out of another, Aquaman led the Atlanteans. Out of another, came the Teen Titans, Robin, Cyborg, Beast Boy, Starfire, and Raven. Out of another, Wonder Woman's mother led forth the army of Themiscyra. Out of another came Shazam, leading the Marvel family. Out of another came the Green Lantern, leading the Lantern Corps, along with every color. Out of another came Young Justice, along with Supergirl and Batgirl.)
Deadpool: Now this is the greatest crossover in history!
Thanos: Oh, phooey.
(later)
(the entirety of Thanos's army is dead, and only Thanos is still standing)
Superman: (lands right in front of him) Just give it up.
Thanos: I will never quit. I am inevitable.
Deadpool: (sitting off to the side, eating popcorn) C'mon... say it... say it...
Batman: (suddenly snatches the Gauntlet) And I am Batman.
Deadpool: (fanboy squeeing) He said it!
Batman: (puts on the Gauntlet and punches Thanos across Wakanda)
Thanos: Ugh. (Superman suddenly appears in front of him)
Superman: I believe in truth, but also justice. (picks up Thanos and shoots heat vision down his eyes, incinerating his brain)
Deadpool: See, people, this is what happens when ol' Supes actually cuts loose.
Deadpool: So, in conclusion, there were many different ways Infinity War could have gone down. And these were simply the best. Ain't that right, Doc?
Strange: I get it, I gave up too soon!