It's not really a theory, more along the lines of a fan-written ending.
So, Saitama fights God in a one-on-one battle. Saitama leads with a serious punch, realizing the gravity of the situation, and this heavily damages God. However, he also begins to put a limiter on Saitama, which makes him weaker. Basically, the entire fight would be a weakening God exchanging blows with a weakening Saitama, ending with Saitama, now no stronger than he was at the start of his training, finishing it off by throwing a punch all the force he can muster. God's dying words are that now he'll never be as strong as he was before, that his rebellion costed him his strength. Saitama says he's wrong, and that if he broke his limits before, he'll just do it again.
He gets back to the other heroes, and Genos notes this difference in power. Saitama is now... weaker. A lot weaker. However, Saitama tells him what he told God, and it ends with Saitama finally, and kind of at an awkward timing due to all that's happened, being hailed as the strongest hero.
Then there's an optional timeskip 3 years later, where Saitama's back at full strength due to training again, which proves he was right about what he said to God.
That's my weird ending for it. I literally thought it up the other day.