I finished the anime but it left me feeling underwhelmed. I'm feeling like Saitama where I'm disappointed when I destroy everything in one punch. I'm waiting for when he faces a worthy adversary. I want to see Saitama bleed. In the 12 eps, the scene I enjoyed most was Saitama training with Genos.
If you want to see Saitama bleed; you are missing the point of One Punch Man.
It's a deconstruction and parody of Super Heroes as a genre. Saitama is; of course; Superman. That hero with no defined limits that is all but invulnerable. Except taken to an extreme, on a character with a very, very different opinion on things.
IIRC, the author wants a 2nd season, but yeah, it's Madhouse's call, because I don't think any other studio would be able to handle it.
I think a 2nd season might be possible; but they might need to wait until Fall 2016/Winter 2017 to start production, unless the rate of the manga/webcomic increases as a response to the popularity of the anime.
But; and I never thought I'd say this; I actually think good filler might be the answer. Work with the mangaka and create eps that focus on
other heroes as filler. Especially given the Hero Hunter arc; I can see potential for a filler around Bang in his prime. Metal Bat might be another candidate seeing as he has a decent role in the Hero Hunter arc too.
Of course; they key word is WELL DONE. Good filler tends to come from working with the source. Bad filler is worse than doing nothing. Good filler can actually enrich the story.
There wouldn't even need to be much filler to make a 12 ep season when you throw in the Hero Hunter Arc; which would last a good few eps. Especially since there's also the meeting with Fukubi before that.