The manga of One-Punch Man is all about how Saitama's unbeatable power makes everything boring to him, and sets him on a search for opponents that might actually pose a challenge - but what if that ultimate opponent has been at his side all along in the form of Genos?
One fan theory suggests that Saitama's final nemesis may actually be Genos, his cyborg protege who has been so eager to learn the secret of his strength. While much of the evidence towards this idea is circumstantial, it comes together to paint a very different picture of Genos and his life than the one that fans have been offered so far. Genos' backstory has only been provided by his own account, and given the nature of his cyborg transformation, his memory of events isn't necessarily trustworthy. This theory doesn't even require that Genos be lying, or evil in any way - just that he doesn't know the truth.
Genos is searching for the "Mad Cyborg," a rogue robotic being that destroyed his hometown and killed everyone but him. The theory proposes that Genos is, in fact, the Cyborg which he seeks, and that he was the sole survivor because he was the perpetrator of the attack. After all, the Mad Cyborg's attack was attributed to a flaw that occurred when placing a brain into a robot body, and by all indications, the only human part left of Genos is his brain. For this to be true, it would require that Dr. Kuseno - the man who found the dying boy and turned him into a cyborg - was lying about the circumstances in which he discovered Genos.
Genos The Mad Cyborg
The chain of events would be that Dr. Kuseno constructed a cyborg body and attempted to place Genos' brain into it before the village was destroyed rather than after, and something went wrong in the transfer which caused Genos to wipe out everything around him. Kuseno could then have blamed himself for the disaster, and made up the version of Genos' past that involved a third party, ensuring he had no memory of the massacre.
In this theory, Kuseno's cautions and warnings to Genos aren't just general fatherly concern, but rather attempts at keeping him from killing again. Kuseno sees Genos' hero career as a chance to make up for the harm they first did, and passively encourages it while still worrying Genos could once again lose control. His moniker, "Demon Cyborg," even seems to hint at the merciless nature with which he approaches foes, and how that might make him similar to his target.
Additionally, Genos' quest for vengeance against the Mad Cyborg has led him to seek power wherever he can find it, incorporating parts from defeated enemies and gaining increasingly monstrous modifications. At the present, his physical form doesn't look all that different from the nearly Monsterized Garou, with smooth black armor and spikes standing out along his arms. While taking parts from villainous robots and cyborgs may well increase his abilities, it also poses some risks - what if these components aren't entirely purged of the villainous qualities of their former owners? After all, the Monsterization process is still shrouded in mystery, particularly as it applies to robotic and cybernetic organisms. It's very possible that some form of malice or evil energy remains within the pieces he's used to upgrade himself.
The Hidden Evidence
One of the most interesting pieces of evidence for Genos' eventual transformation into a monster is found in the original, digital version of chapter 86. As Dr. Genus of the House of Evolution explains the idea of humans turning into monsters to Zombieman, a spread of formerly human monsters can be seen. Hidden among them is Genos' face, obscured by the much larger and more prevalent creatures on the page (central in the image above, below the outstretched fist of Fist Fight Djinn.) This page also states that Monsterization is often driven by "An explosion of negative emotions" - something that would almost certainly happen if Genos were to learn he is the Mad Cyborg.
There are some arguments against this theory, however. Early on in the manga, another robotic/cybernetic hero, Drive Knight, tells Genos that the Mad Cyborg is affiliated with Metal Knight, another hero who operates exclusively through remote control drones and is particularly fond of performing weapons tests. Of course, Drive Knight could always simply be flat-out wrong or misinformed; he seems to be searching for this Mad Cyborg as well, and may be on the wrong trail. It's also possible that Drive Knight is lying, and that this is done as some kind of strategy to trip up Metal Knight in the arena of hero politics. Drive Knight does combine with Genos at one point, but has not proved himself particularly trustworthy, as he abandoned a self-destructing Genos despite the risk to Blizzard, Bang, and several other heroes.
As for the spread in Chapter 86, the panel was altered when the chapter went to print, something that's not uncommon for One-Punch Man as it prepares to release paperback volumes. In the print version, Genos' face was removed from the panel entirely. While some argue it was a mistake that was corrected, others claim it was done because it too heavily foreshadowed Genos' eventual fate and gave away the twist. If Genos' quest for power leads him to take in monster components and transform fully into a monster himself, then there would only be one hero capable of stopping Genos before he could destroy another town: Saitama.
Saitama's Final Challenge
Rather than Saitama's greatest opponent being someone with strength superior to his, the fact which would make the fight difficult (and interesting for Saitama) is that even in such a scenario, Saitama wouldn't want to just destroy Genos. They've spent so long together becoming friends that killing him even after he'd turned into a monster would be unacceptable.
Saitama's challenge would be to defeat Genos and the overwhelming power he wields without simply vaporizing him the way so many other major villains have fallen. The genius of this reveal isn't just that it gives Genos' quest a cruelly ironic twist, but that it turns the fan-favorite character into Saitama's final challenge. The ultimate opponent for the too-strong Saitama in One-Punch Man wouldn't be someone who challenges him physically, but - in Genos - someone he can't bear to obliterate.