Black Panther's worldwide box office currently ranks as the 10th highest of all time at $1.3 billion, which should tell you all you need to know about how finite the ending of Avengers: Infinity War will prove to be.
The odds the King of Wakanda vanished into dust forever before he got a sequel is less than zero (like, 1.3 billion less), which means everyone else who disappeared when Thanos snapped his fingers in the movie's final sequence can't be considered permanently dead either. They are Marvel dead, which means they will likely come back to life soon enough. But we're not concerned with Marvel's bottom dollar; we care about the story. So did anyone who bit the dust in the movie actually leave the MCU forever? Here are the three important characters who are likely gone for good... and one surprising one who probably isn't.
The brave and noble guard of the Bifrost Bridge was all-seeing and all-knowing, but he was caught off guard when Thanos and his children attacked the Asgardian escape ship. While the fates of other Ragnarok characters like Valkyrie, Korg, and Miek are unknown, we saw the wounded Heimdall make one final sacrifice calling on the Bifrost to get Hulk out of dodge safely, drawing attention to himself in the process. We can safely assume that sword he subsequently took to the heart definitely and permanently ended his life, since the act did not occur in the presence of any of the Infinity Stones.
Hypothetically, the involvement of the Reality Stone in a death scene like this one could suggest that what we saw was only a manipulation. Similarly, we can rule out Heimdall's death being overridden by the Time Stone, as Mordo warned in Doctor Strange that violating with the laws of the universe on such a grand scale risks ripping apart the fabric of the universe itself. (In other words, it doesn't seem anyone could safely go that far back in time to bring back Heimdall without reaping something much worse.) Finally, Heimdall's death took place long before anyone had the Soul Stone, whose powers are connected to another plane of existence but are not yet fully understood. As such, we can't foresee anything that would bring Heimdall back to life.
...UNLESS he, with his orange eyes and ability to see trillions of souls, has an intimate connection to the Soul Stone we don't know about, and manages to come back thanks to his own inborn powers. This is Marvel after all.
Employing the same logic we used to deduce that Heimdall is dead for good, we can say with confidence that Loki's second death scene (remember, he "died" in Thor: The Dark World) is the real deal. First of all, none of the Infinity Stones that might be used to undo Loki's death were in Thanos' possession when he killed the trickster. Considering Thanos had an easy time defeating Hulk, who not even Loki could stop, there's no reason to doubt he was fully capable of snapping Loki's neck with his bare hands. Loki went down swinging, trying to save his brother's life, but he definitely went down.
...UNLESS the God of Mischief used his classic misdirection/multiple persona tricks and Thanos only thinks he killed the real Loki. This is Marvel after all.
Vision was always going to be a major target of Thanos, but his importance to the story was so great it led to the massive attack on Wakanda, where the entire stand was about keeping him safe. Dora Milaje aside, things never looked promising for Vision, even after the other Avengers refused to sacrifice his life and Shuri found a way to separate him from the Mind Stone. Time and the Mad Titan were against him, and too much was on the line to let him live if and when the time came for Scarlet Witch to do what needed to be done. But it's Vision's second death, after Thanos used the Time stone to bring him back, that has us convinced he's truly dead. Because Thanos not only ripped the stone from his face, he tore his entire head out. And just like Loki, who turned totally gray, all of the color and life from Vision disappeared forever.
...UNLESS all of the other things that brought him to life originally—the pieces of Ultron, Jarvis, Tony Stark, and Bruce Banner—are enough to reignite his light. This is Marvel after all.
Those are the three we'd bank on never seeing again (outside of a flashback), but there's one character who died who can't be included on that list.
The most important death of them all, the one that allowed Thanos to acquire the hidden Soul Stone, was his "daughter" Gamora's. Thanos threw Gamora from the top of Vormir, sacrificing the thing he loved most for the power to gain the stone. This is exactly why we're not sure she's dead for good. Something funky definitely happened there. First off, he was instantly transported to an eerie body of water after Gamora died, much like the locale he found himself enjoying after "killing" half of the universe with the snap of his fingers.
Waiting for Thanos there that second time was young Gamora. Is she trapped inside the Soul Stone? Is she not really gone? What was Red Skull's cryptic warning to him all about? Gamora might very well be dead, and her soul might merely exist in a type of ancestral plane like we saw T'Challa and Killmonger visit in Wakanda. But we can't be sure. This is Marvel after all.
What do you think? Who is definitely dead forever? Who isn't? Tell us in the comments below.