There are several key moments in Avengers: Infinity War. Arguably none was bigger than the moment Thanos and Gamora visit Vormir for the Soul Stone. There, Thanos does an unspeakable act that completely changes the complexion of the Marvel Cinematic Universe.

Faced with the reality that he must sacrifice that which he loves to acquire the Soul Stone, Thanos throws Gamora to her death. It was a heartbreaking moment that underlined the terrible lengths the Mad Titan would go through to complete his vision.

I bring this up because acquiring the Soul Stone could be key to changing the outcome of Avengers 4.

There have been multiple theories about how the Avengers will go back in time and acquire the Infinity Stones. But, when faced with making a sacrifice to acquire the Soul Stone, what’s going to happen?

Again, in Avengers: Infinity War, Thanos sacrifices Gamora. What can the Avengers do to get their hands on the Soul Stone?

Just so we’re clear, here’s how Red Skull explains what must be down in order to acquire the stone:

“To ensure that whoever possesses it understands its power the stone demands a sacrifice. In order to take the stone you must lose that which you love. A soul for a soul.”

Cap sacrifices himself

Captain America sacrificing himself makes the most sense and will likely carry the biggest emotional punch. Plus, Chris Evans just announced that he’s essentially done being Captain America in the Marvel Cinematic Universe, suggesting the character gets a conclusive ending in Avengers 4. There’s nothing more conclusive than death.

Out of the Avengers who survived Thanos’ snap in Avengers: Infinity War, Captain America barely got any screen time. That’s going to change in Avengers 4, and it could mean he goes out in a memorable way. Captain America has selflessly sacrificed himself before—he jumped on a dummy grenade in Captain America: The First Avengers.

When the choice comes down to saving half of the universe and saving himself, he won’t hesitate to save half of the universe, even though he repeatedly said he doesn’t trade lives.

Tony sacrifices himself

Tony Stark kicked off the MCU and he could make his grand exit with the ultimate sacrifice. It would be fitting for a character whose ego barely fit in his nano suit.

He has embarked on a number of suicide missions before—he flew a nuke through a wormhole in The Avengers—so he has flirted with death before. Not to mention he flew to Titan to face Thanos head on rather than returning to Earth.

What if Stark gives his life to bring back the millions (billions?) who perished in Avengers: Infinity War? It would be a nice narrative conclusion for a character so terrified of death.

Loki sacrifices himself

Is Loki dead? It’s hard to truly say, considering the trickster has faked his death before. It sure looked like Thanos choked the life from Thor’s brother, but we’re not so sure Loki wasn’t pulling one last con.

At this point, Loki has redeemed his misdeeds by helping out Thor and otherwise actually caring about doing the right thing. But his narrative arc could end in epic fashion if he makes the ultimate sacrifice for the greater good.

Thor has already mourned his brother (twice), and the third time would surely sting the most. But if it’s in the name of saving the universe, Loki would earn the fame he’s after while completely redeeming himself as one of the good guys.

Nebula sacrifices herself

Since her debut in Guardians of the Galaxy, Nebula has been portrayed as angry and violent, with no emotional complexity in between. But starting with Guardians of the Galaxy Vol. 2, her thick exterior was peeled away to reveal a more nuanced and vulnerable character.

We know she desperately wants to kill Thanos. Short of that—and with the potential to bring Gamora back from the dead—her biggest contribution might be in getting the Soul Stone. To do that, she could sacrifice herself on Vormir and help get the Soul Stone before Thanos does.

Thanos sacrifices himself

This one is far-fetched and has about a 1-percent chance of happening, but what if? We saw how heartbroken Thanos was after killing the only person he ever loved. Avengers 4 could see him unable to cope with his grief and reverse what he did by sacrificing himself.

We also know that in a twisted way Gamora very much cares about Thanos, despite his diabolical plan and her hatred for his views and how he treated Nebula. But the sacrifice could further humanize Thanos and prove him to be one of the most complex villains we’ve ever seen.