Spoilers ahead for Infinity War. Avengers: Infinity War will answer a lot of questions that Marvel fans have been asking leading up to the movie, like where the Soul Stone was located all along. One question will still remain after you see the movie, though, and that's why Valkyrie isn't in Infinity War. Everything leading up to the new film, namely the end credits of Thor: Ragnarok, set up the latest Avengers movie to include Valkyrie (Tessa Thompson). But she's totally missing from Infinity War, and that absence goes largely unaddressed.
The most logical, unexciting reason for Valkyrie being M.I.A. could probably be chalked up to the fact that Thompson simply couldn't fit the film into her busy schedule, what with her roles in Westworld, Annihilation, and Creed II. As for what Valkyrie was doing during the massive showdown against Thanos, you can use your imagination. Thompson herself has a pretty reasonable explanation for where her character might have been during the world's end: she's busy getting a drink. The actor's funny excuse came in response to tweet asking why neither Valkyrie nor Hawkeye appeared in any of Infinity War's trailers. "They are at the bar ... ?," Thompson wrote back. The ellipses at the end of that explanation seems to imply a "duh," which is so Valkyrie of her.
It's a shame that Valkyrie couldn't show up in Infinity War, and most viewers would probably admit to waiting for her arrival throughout the movie. Part of what made Ragnarok such a hit was the badass hero, after all. We all loved her backstory of being a former member of the Valkyries, the all-women elite army who protected the throne of Asgard not unlike Black Panther's Dora Milaje.
You never get to see Thompson's character actually in action with the Valkyries, except for a sad flashback in Ragnarok which shows Hela (Cate Blanchett) conquering the group of female soldiers riding white winged horses, but you get a good sense of her fighting skills when she goes hand-to-hand against Loki in one scene. With those skills, Ii would have been great to have her on the ship in Infinity War when Thanos arrived.
The fact that Valkyrie wasn't on the ship is something of a continuity issue, seeing that Ragnarok ended with Thompson's character joining the Asgardians when Thor saved them. Considering how much Valkyrie loves to drink, maybe it isn't actually such a stretch that she literally abandoned ship when she saw a floating space bar.
As for Valkyrie's future in the MCU, that's another unknown. Since Chris Hemsworth's contract with Marvel ends after Avengers 4, it's possible that you'll have seen the last of the Asgardians come May 2019. Out of the 16 Marvel movies set to follow Infinity War, none of them are expected to include Thor or any characters from Asgard. That doesn't necessarily mean that a fourth Thor movie is out of the question, though.
Last January, Hemsworth told USA Today that he'd spent New Year's Eve in Australia with Ragnarok's director, Taika Waititi. Hemsworth recounted the coworkers and friends' night together, saying, "We gave cheers to Thor 3. But it also was like, 'What could we do with four? We could do this and this... We were throwing ideas around." It sounds like Hemsworth might be persuaded to extend his Marvel contract in order to work with Waititi, because the actor told USA Today, "[Ragnarok] was one of the most enjoyable creative partnerships I’ve ever had."
Should Hemsworth and Waititi team up for another Thor movie — which they absolutely should — they'd also need to make sure that Thompson has time in her busy schedule to join the cast. She's so greatly missed in Infinity War.