Well, I'm also expecting them to delve more into what causes monsterization. From what I gathered in the first episodes of the first season, people turn into monsters according to whatever obsessions they have (like that car man or the crab monster Saitama first met). After Saitama's explanation of his training and how genos and the scientist themselves pointed out it was in-universe unrealistic (in the sense that such a training regimen would never make him that strong) I've been entertaining the idea that heroes like Saitama (who obsessed with being the strongest) and watchdog man (who takes roleplaying a watch dog too far) are actually monsters all along. Which puts OPM on route to having the other heroes fight Saitama since they're the only people who can put up a good fight (either by coming close enough in skill to get him or due to Saitama holding back because of his moral code).
Though I guess there'd be no problem with Saitama being just a device that humbles people arrogant enough to think they have no match. But judging how that aspect was handled with Suiryuu (who came off more like a guy with an arrogant streak but a set of values disguised by a cavalier attitude), Saitama might end up coming off as a moralizing nag rather than a guy who sets the record straight more often than not.