Garou from OPM might also work well. His ideology of being a 'human monster' and his constantly evolving strength could make him a formidable foe for Saitama. Their clash could be not only about physical strength but also about their different views on what it means to be a hero or a monster.
Another interesting concept is a crossover where the villains from the OPM world are a threat to Saitama's own world. Saitama then has to team up with the OPM heroes to fight back. This could lead to some really cool fight scenes and interactions between different characters.
To write a character - driven story about Saitama, focus on his boredom. Show how his extreme power has made his life lackluster. For example, you could write about his attempts to find a hobby that actually challenges him. Maybe he tries painting, but he's so good at it immediately that he gets bored. This exploration of his search for something to fill the void in his life can be a great character - driven story.
Tornado of Terror would likely be one of the first to try and take on Evil Saitama. She's powerful and confident, but she would soon realize the extent of his power. Mumen Rider, despite being weak, would still stand up to Evil Saitama out of a sense of justice. He would be quickly defeated but his act of courage might inspire other heroes to step up.
One idea could be Saitama facing an enemy that can copy his strength but not his speed. This would create a unique dynamic as Saitama has to use his wits more than just his punch. Another plot could involve Saitama traveling to a different dimension where his power is nullified and he has to start from scratch to regain his strength. And perhaps a plot where Saitama has to train a new hero who is having trouble controlling their own power, similar to how he was once a regular person trying to become strong.
Yes, the OPM manga has come to an end. The creators completed the narrative, providing closure to the fans. Although some might have hoped for more, what was presented was a comprehensive and conclusive tale.