I'm gonna be using this as the basis on the idea. Also visit his channel it's actually funny)

Dante is saddened by the experience he had with Donny, some kid he saved and who was inspired to be like Dante but this path got him into trouble and he comes off as just a douchebag who pisses off the demons with his "jokes" rather than Dante who actually has wits with his jokes and it sometimes gets a laugh out of his foes.

Dante gets fed up saving him for the upteenth time and Dominic pushes Donny over the edge by insulting his humanity calling him weak and saying maybe if he was a demon he could actually be something. And that's what he does. Donny gets demonic powers and starts fucking shit up where he shouldn't be getting the Devil May Cry family involved and stopping Donny from killing a human man who isn't actually Mundus just a dickhead of a human that Dominic slaps for talking to them like garbage( because he's a boss ass bitch who doesn't take shit, not even from his own family)

Dante kills Donny which starts up his long gone depression after he believed he had killed his brother after the events of DMC 3. The man who was posing as "Vergil" was a demon manipulating him who Dominic kills. Dante's demeanor noticably gets more depressing as time goes on which his whole family notices.

Dominic visits his brother a lot more showing care for his brother that was never shown before showing how different he is from Vergil. He's willing to show emotion and compassion to his family. Dominic helps Dante out through his depression and the revelation that his wife Blake was pregnant broke Dante out of his depression fully. After nine months his son Brian Belladonna was born someone who can carry on his legacy (and don't worry he will still be a mentor to Nero as he was in four)

Brian would grow into a carbon copy of Donny giving Dante the motivation to be a better mentor this time over and reboot Dante isn't a fucking douchebag.

Also Brian's powers are a mix between Blake's semblance and Dante's doppelganger sort of like Noob Saibots in Mortal Kombat and where he was during the fifth game, he was in another part of the world dealing with other instances of the Qliphoth rooting itself in the earth which is why the rest of the Sparda family isn't there to help Nero fight Urizen/Dominic.

And now we can get Sparda brothers who don't try and kill each other and do this

But God damn do these two old men have good chemistry. Stay safe y'all.