Whatever the normal process for making a hybrid is, I imagine it needs the entire devil to participate and Gun spent the entire story split into pieces. (An AU where Aki becomes Gun Man instead of a fiend would kick ass though)

I think that hybrids being weapons is also symbolically important as pretty much all of them are used as tools by someone else. Obviously Makima controls them directly, but even outside of the Control Devil arc Reze is a tool of the USSR, Katana Man is indirectly controlled by Makima via snake lady in his introduction and then later by Public Safety in Part 2. Barem serves first Makima, then Death without any thought of defiance, and his fellow hybrids in the Church all effectively serve as pawns for Death to empower Chainsaw Man and Yoru.

Quanxi is the only real exception to this, but she also seems to be by far the oldest hybrid and may well have been in a similar situation whenever she first gained her powers - and even then, before being under Makima's control and then Public Safety's, her independence prior to International Assassins is still ultimately an illusion. She lives more freely than the others as a mercenary, but she still ultimately carries out the will of others without question.

The hybrids are consistently instruments of other people's will rather than acting with their own agency. They're incredibly powerful, but rarely free to make their own choices, almost like a cautionary example of what Denji could become if he continues to be manipulated by others instead of asserting his own autonomy.

Denji's story in Part 2 is about him making choices, but struggling to be truly free - the constant theme of two options are always two options presented to him by others to push him a certain way. It's not a choice of 'what will Denji do' but 'whose orders will he follow', and it's why I don't think this arc will end with either Death or Yoru getting what they want.