Well, Mugen Visual Novel is a concept that fuses two different things. Visual novels are known for their story - driven gameplay, with lots of text, choices, and different story paths. Mugen, on the other hand, has been associated with creating custom fighting game content. In a Mugen Visual Novel, perhaps you could have a story where the outcome is affected by fighting elements. For example, characters might battle to progress the story, or their fighting abilities could be part of their character development in the narrative.
One idea could be a rengoku fanfic where he trains a new group of young slayers. It could focus on his unique teaching methods and how he instills his values in them.