TheRealBrandini97's theory posits that it's no coincidence which original Avengers are saying the phrase. The four repeating it are the ones who will make the ultimate sacrifice and lay down their lives to undo what Thanos has done.

There are a couple fan theories going around about how Cap will use Thor's hammer Mjölnir, based on him interacting with it in Age Of Ultron. But user webdevnoobs thinks the idea is that Captain America will only be worthy to wield the godhammer when he's his original, scrawny self: Steve Rogers.

This one's a doozy, with jlefrench positing that Thanos (and Doctor Strange, who can see all possible futures) realized big bad planet-eater Galactus was coming. Maybe not the best reasoning but Thanos decided to "save" the universe by dimming its energy (i.e. killing half of it) so Galactus couldn't snack.

CaptainCayden2077 predicts a specific plot twist in which everything seems set up for Captain America to sacrifice himself (seems pretty in character and all), but when it comes time to get the Soul stone, Tony's the one who'll take a hit for the team.

This one ties in Ant-Man's Quantum Realm travels, Doctor Strange's visions of possibilities, and Loki's portal. redstar6486 believes that thanks to Ant-Man showing up, Strange realizes there's a way to connect universes and shuffle folks from one to the other, not messing with the timeline but creating a universe where everyone's alive again. Only problem is he'd have to hope Ant-Man comes in from an alternate universe where the other half of the population was destroyed, otherwise you're going to have a lot of double-trouble.

Another from webdevnoobs, but this one treats Endgame like a heist movie. Instead of zipping through time and space to yoink the actual Infinity stones the team will just copy their energy signatures, allowing them to build a weapon to harness their power without altering the timeline. Bonus: it explains some of the quirks in Infinity War.

Hosnims goes a little deep into time-travel logic but basically makes a specific point a couple other theories have hinted at: there's time-travel right there in the trailer, in the form of Thanos beaming down to earth. He has the Infinity Stones, why would he need to beam anywhere? Unless we're seeing him before he got the stones. That's right, instead of closing the alien invasion portal from the first Avengers, Tony lets 'em come on in so they can whomp him before Thanos starts collecting.

ahumanmadethis has fan-fingers crossed that a) the stones will need to be destroyed for the good of the universe and b) that the shock-waves/release of their powers will 'unlock' those powers in creatures the universe over, just like the Mind Stone unlocked the powers of Scarlett Witch and Quicksilver (and now that Disney owns the company that can use the X-Gene and the rest of Marvel, maybe they can call mutant powers for what they are).

Enervata steps on the toes of previously mentioned theories about the multiverse and universe shuffling, but also posits that we, the audience (and also Doctor Strange, Ant-Man, and Captain Marvel) experience the multiverse where everyone's alive in Universe-102A, but in the universe where Thanos snapped...everyone's still gone, and it's a noble story of accepting and moving through loss and sadness, whether through permanent separation or sacrifice. A perfect case of having your fan cake and eating it too.

Ok, one final (and extremely thorough) twist from mysticsaidge on the "alternate universes" save that addresses the previous problem of Ant Man traveling to the perfect inverse universe: When Thanos snapped his fingers the universe split into two new universes, each a mirror image of the other, in terms of who died. Same deal with Ant Man traveling through as messenger, but this theory gets excruciatingly specific as to who gets re-dusted when the universes are merged back together (instead of other theories' transferring people). And it just happens to note whose contracts are up.

Infinity War wasn't just a movie title. RahulVijh posits that before Strange became dust in the wind, he set up a time loop to defeat Thanos similar to how he defeated Dormammu in Doctor Strange, despite repeatedly getting killed. In this case, everyone's stuck in a giant repeating time loop until wildcards Ant-Man and Captain Marvel can break on through.

LoL-Guru gives Strange a lot more credit as a master manipulator of time, using the Time Stone's ability to zip it to a future where Thanos can't get it. Too bad the dude's immortal and can wait it out.

The nice thing about the thousands of fan theories floating in the internet ether is come April 26, they'll all be proven right or wrong to varying degrees, none of which will affect anyone's enjoyment one bit.