Warning: minor SPOILERS for Thor: Ragnarok are ahead!
Asgard is filled with mighty warriors, and once upon a time, the Valkyries were the best of the best. Thor looked up to this elite team in his younger years, so when he saw Tessa Thompson's Valkyrie on Sakaar during Thor: Ragnarok, he was understandably starstruck. This was the first time those characters had ever met, but in an earlier version of the threequel, director Taika Waititi thought about featuring a flashback sequence depicting an earlier meeting between the two, and said flashback would have seen them rocking 1980s look. Waititi explained:
Well I wanted this little thing, and if we ever do a Thor 4, maybe we can have it in this next one, but I want to do some flashbacks where Thor was a kid, a fat little kid. There was like an 80s version of Asgard where everyone had massive shoulder pads, and everyone had mullets...Our idea was Thor and Valkyrie meet and he's like, 'Hey I know you,' and she's like 'Ha I remember you,' and then it cuts back to this thing, and he's just this pudgy little kid walking around with a mullet and being picked on by other kids. And Loki's like this little emo goth hanging out by himself. He was like the kid in Harry Potter [Malfoy].
Most of the Marvel Cinematic Universe movies have been infused with humor, but Thor: Ragnarok doubled down on what its predecessors offered, becoming more tonally similar to the Guardians of the Galaxy movies. That being said, the idea of seeing Asgardians decked out in '80s attire a millennia or so years ago and a chubby Thor hits a new level of absurdity. A Malfoy-like goth Loki, on the other hand, doesn't sound as ridiculous given how he turned out as an adult. As for why this flashback scene wasn't included, here's what Taika Waititi said during his interview with MTV's Happy Sad Confused podcast.
The problem was we needed him to not know Valkyire in the film and stuff. And then it became less and less of a thing to have a flashback. We would've been forcing it, so it would've been a situation where we would have been having a pointless flashback.
Plot-wise, Taika Waititi is correct that this flashback wouldn't have made any sense in Thor: Ragnarok. But as a special feature in the Thor: Ragnarok home release or a fun gag video for Marvel's official channels, this is definitely worth spending some money to shoot, especially since there's no word yet on if there will be a fourth Thor movie or if Takia Waititi will helm another Marvel movie. I'd even watch a series of short videos following Thor and Loki's time in Asgardian elementary/middle school during this "'80s" period, continuity be damned. This wish will likely go unfulfilled, but coming back to reality, Thor, Valkyrie and Loki will all be back next year for Avengers: Infinity War.