I'm a pretty big manga geek, though I tend to read more Korean and Chinese stuff. While many of the tropes/plot devices are heavily overdone in the medium, I think they can work very well in adult games.
Take Chinese martial art stories for example. There are typically two forms, the ancient fantasy type and the modern type. In the fantasy version, they are generally a weak peasant who was once a genius amongst their peers. Something happened to them and they become oppressed by another individual or organization. The person in question discovers some artifact/technique which allows them to recover their genius status. They travel around beating up arrogant assholes while being swooned over by many beautiful women. The modern is similar, but its generally based in modern times. The MC usually unlocks some sort of system which allows them to sacrifice or gain something in order to gain skills/abilities. While generally the requirement is to collect money and offer it to the system, in one particular manga the system requires the protagonist to be involved in lewd situations. The other trope is that of the protagonist lived his life, became an immortal, failed to ascend to godhood and went back in time to before he began cultivating.
Theres also the "tower" trope, system, vrmmo, isekai, reincarnation, experience, doctor, alchemy, etc.
Any combination would work for adult games. As many of us have a bit of a god complex and want the girls and worlds to revolve around us. Any of these tropes usually come with overpowered protagonists, causing everything to revolve around us.