WARNING: The following contains spoilers for My Hero Academia Chapter 339, "The Story of How We All Became Heroes, Part 2” by Horikoshi Kohei, Caleb Cook, and John Hunt, available now in English from Viz Media.
My Hero Academia fans will absolutely flip when something from outside the original manga enters the story. Whether it's from the movies or the spinoff series, when such material makes it into the official MHA, it creates an overwhelming feeling of gratification in the die-hard fans who recognize the reference. Such a reaction can be observed when Shirakumo, Aizawa's friend from high school who's expanded upon in My Hero Academia: Vigilantes, appears in MHA proper's 255th chapter, "Hero Hopeful." This narrative interconnectivity makes MHA feel more like the comics it's inspired by and shows how far along it's come as both a series and a franchise.
A similar reaction can be created with Chapter 339 of the MHA manga, "The Story of How We All Became Heroes Part 2," when, Deku received a familiar upgrade to his gauntlets. They now look more like the Full Gauntlet support item he received in My Hero Academia: Two Heroes.
In Two Heroes, when Deku is introduced to Melissa Shield, she noticed the scars on his hands and recommends he wear a support item. She offered him her Full Gauntlet, a support item she's been working on that functions as a high-tech arm brace. It allowed Deku to use 100% of One For All's power up to three times without injuring his arm. He sadly broke it by the end of the movie, but its potency was all but proven.
It's probably for the best that this item wasn't entered into the MHA canon at this point. The movie is estimated to take place sometime between Deku's First Term Final Exams and his time at the Quirk Training Camp. If this is true, then Deku should only be able to safely use about 5% of One For All's power. He says as much in the movie. Even if the Full Gauntlet only lets him use three full-powered punches, having it that early would have broken the narrative in many places.
It's actually in Chapter 306 of the manga, "The Final Act Begins," where Deku began using a support item, not unlike the Full Gauntlet. They share the same color, design, and function as the Full Gauntlet. These "Mid-Gauntlets" allowed Deku to use more of One For All's power without straining himself. They also happen to be imported from the United States. All of these facts seem to suggest that either Melissa or her father David have some part in their development.
The Mid-Gauntlets, however, have some key characteristics that differentiate them from their apparent predecessor. For one thing, they will shatter if Deku tries to use One For All at 100% which technically makes them weaker than the Full Gauntlet, but that's fine as they're only prototypes. They can also be used to reinforce Deku's entire body, not just his arms. In this state, the Mid-Gauntlets are best used for improving Deku's general performance, but if he wants to go all out, he may need to give Melissa a call.
Unfortunately, Deku broke his Mid-Gauntlets during his vigilante stint. Specifically, the left gauntlet gets completely destroyed during his fight with Lady Nagant. Following this bout, the Gauntlets and the rest of his outfit suffer from serious wear and tear as he continues fighting villains without rest.
It's only when Deku returned to UA High that he's able to get his Gauntlets fixed. In Chapter 339, he visited Mei Hatsume at her workshop and requests that she repair them. She seemed to refuse at first, citing the difficulty, but then she casually created similar support items while giving a moving speech about being in the Support Class. If Mei can't fully replicate the original, then perhaps Melissa still has a chance to appear in the story with the finished product.
The Gauntlets are a good upgrade to Deku's arsenal, but they serve an even greater purpose. Their existence creates hope that Melissa and other non-canon characters could someday show up and help the main cast out. There might even be a chance for Vigilantes' protagonist The Crawler to at least make a cameo. If Chainsaw Man can show up for a cameo, surely one of these in-universe characters can, too.
Wherever the Gauntlets lead, simply having them in the story is a great pleasure. Their presence serves to make MHA and all the stories set in its world feel more connected than ever before. They also remind audiences that their favorite characters from other MHA works are somewhere out there; whether they show up, however, is a different story. As they do with Deku, the Gauntlets provide the audience with a greatly appreciated comfort.