Thanks to things like the Marvel Cinematic Universe, fans of various genres now tend to view things through the prism of "it's all connected," even when such connections are more wishful thinking than keen insight. Case in point: some anime fans are now subscribing to a theory that tries to connect the mythos of My Hero Academia with the mysterious backstory of One-Punch Man.
As you can read in the Reddit thread below, the premise of the theory is that the world of One-Punch Man is actually the same world as My Hero Academia, only further into the future, long after the tale of Izuku Midoriya that we're currently following in the latter series. Admittedly, it's not the worst fan theory we've ever heard.
[One Punch Man & My Hero Academia] I think Saitama possess One For all from an All Might Descendant. from r/FanTheories
The theory frames the strange world of OPM's superpowered heroes, villains, and creatures as the advanced version of MHA's society, where Quirks have been around long enough to be normalized and routine. OPM's world of numbered cities and districts that are constantly being rebuilt are a reflection of a society having adapted to Pro Hero and villain battles, and learning to cope with the constant destruction, and ditto for the various organizations of heroes, villains, or strange science that exist in OPM.
The theory really gets juicy when it seeks to explain how OPM hero Saitama was secretly given the quirk power of MHA's iconic hero, All Might. OPM began with the story of how Saitama defeats a crab creature named Crablante to save a kid, and in this theory, during Saitama's initial stint of getting his butt kicked by Crablante, there was a moment that's yet to be revealed where the future successor of All Might and Izuku Midoriya's "One For All" Quirk passed the power on to Saitama, resulting in him manifesting the power to suddenly punch Crablante to pieces, and do the same to so many villains after that.
While My Hero Academia has done a great job fleshing out and expanding the mythos created by Kohei Horikoshi, One-Punch Man has been a lot more fast and loose with its mythos, hardly explaining anything concrete at all about Saitama's powers. In fact, part of the comedy of One-Punch Man is that Saitama has the delusion that his regimen of 100 push-ups, 100 sit-ups, 100 squats, a 10-kilometer run, eating right, and mental fortitude is solely responsible for his one-of-a-kind power. Similarly, the world of superheroes, supervillains, cybernetic warriors, animal-human hybrids, and everything in between is hardly explained or even referenced as being anything extraordinary. That ambiguity leaves a lot of room for theories like the one above to be applied, no matter how specious it may be.
Truthfully, it would be pretty cool to find out that One-Punch Man is a sequel to My Hero Academia, but since the former (2009) was first released a full half-decade before the latter (2014), it's a hard view to sustain. Not to mention, all the obvious legal separations.
One-Punch Man is currently in production on Season Two. My Hero Academia is currently in production on Season Four.