That might be a good pitch for an anniversary game. Nero investigates his parents and looks into what happened 20 years ago.Maybe a new character like Nero's mum?
I really like this concept of yours BUT please don't give Capcom any ideas that we know they won't do any justice.
Just my 2 cents.
OkAbout half the story he can summon the demon that killed his parents and after a boss fight defeats him. But soon afterwards a horrible truth will be revealed.
The next question would be how he interacts with the cast of DMC...specificalky the agency (Morrison/Patty/lady/trish)