One-Punch Man would represent a significant challenge if he fought with any heroes or villains from the Marvel Universe. As his name suggests, Saitama can defeat anyone with a single punch. However, he made a surprise cameo in the pages of Marvel Comics - without doing any punching.
One-Punch Man first debuted in the webcomic of the same name and has starred in both a long-running Manga series and an anime series. What makes Saitama such a great character is that he gains his one-punch powers from just doing mostly normal exercising, doing 100 push-ups, sit-ups, and squats every day, as well as running and eating healthy. While the routine makes him go bald, Saitama can take out any villain with a single punch. The series is humorous, but his also incredibly action-packed. Marvel must be fans of One-Punch Man, because, in an issue of All-New Inhumans, Saitama makes a surprise appearance... sort of.
In All-New Inhumans #5 by James Asmus, Andre Lima Araujo, Andres Mossa, a new character is part of the People's Defense Force of China. The unnamed character looks strikingly like Saitama. He's bald, has a similar cape, has the same colored outfit, and the exact style of boots. He doesn't have sleeves, a belt, gloves, or have One-Punch Man's powers, but there's no denying that the character's look is directly inspired by Saitama. It's too much of a coincidence. I mean, look at him.
The unnamed character appears to have powers of controlling wildlife, as he's got a bird and a tiger under his control. So, the inspiration is more about his look, not the character's overall powerset. It's possible the character is a redesign of the metahuman known as Most Perfect Hero, who wore a yellow outfit (with a mask) on China's Defense Force, before being killed in his only appearance in Mighty Avengers #27. But, whoever the character is now, it's clear that One-Punch Man was the primary design inspiration. Popular cartoon/anime making cameos in Comics isn't anything new. Dragon Ball Z exists in the DC Comics Universe. On the flip side, Marvel and DC heroes exist in the My Hero Academia Universe. It would be incredibly cool to see One-Punch Man actually show up in the Marvel Comics universe - as his powers would be a true test for all of Earth's Mightiest Heroes.