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.
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.
Their friends would likely be a mix of shocked and excited. Raven, being the more reserved one, might initially be a bit concerned about how this will change their dynamic as a team. But she would also offer her support in her own quiet way. Cyborg would probably be the most excited, already thinking about building all sorts of baby - related gadgets. Starfire would be overjoyed and would immediately start planning baby showers and knitting baby clothes. Robin would be more practical, thinking about security measures to protect Terra during the pregnancy and the new family once the baby arrives.
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.
I'm not sure specifically what 'november terra' refers to in this context as the term is rather unclear without more background. It could potentially be a made - up name for a place, a character, or something else entirely.