Nero is quite troublesome to write. Bingo said something about Nero wouldn't care about human if it wasn't for Kyrie's love before didn't he?
Okay so my idea for Nero would be turning him to be someone who is cynical and doesn't have any strong opinion on both demons and humans. He is a child grew up with no real genuine love. His mother left him at orphanage, he never knew who is his father. People at Fortuna would be superficially nice to him or just indifferent. If Vergil is a prideful, narcissistic man who would do cruel and selfish acts to satisfy his ego, then Nero would be someone who doesn't care about his self so much. No one cares for him, Nero feeling like a fleshy ghost in Fortuna. So he would be quite apathetic towards humanity I guess. If Nero got involved early with Order then he'd be a very efficient soldier, following every orders. Kill demons? Done. Kill the ones who disagreed and tried to rebel against Order? Done. Because being Order's tool gives him a purpose yada yada
I'd change Kyrie as an outsider from Fortuna, symbolizing the new change happening to Nero. She would come along with Dante as work partner to investigate or something I don't know. Or no Dante involved in dmc4 at all eh. Kyrie would be disturbed with Nero's apathy and attempt to persuade him and challenge Nero's worldview by helping Fortuna citizens, who are strangers to her. Kyrie needs a reason to personally care about Nero's mental state but I'm running out of idea. Nero at first doesn't understand why she helped strangers without ulterior motive. But seeing the people she helped came back to repay her kindness and assist her makes Nero slowly come to a realization that he's being negatively influenced. Having humanity means you're capable of change, you can grow, you can inspire others to be better, but you need to be in a positive environment in order to grow as well. Kyrie inspired Nero, starting his blooming affection for her here. Nero switches side and rebels against Order. Order is defeated. Happy ending the end.
My idea is still quite clunky, doesn't seem to relate with dmc theme... Is it too long? It's not like I'm going to write a fanfic for this soon.