I had this idea for DmC 2, where Dante is on a train that kind of travels through astral planes (Phineas informs him of its existence) and it's heading for Purgatory (Angel version of Limbo) And I thought it’d be cool to have the carriages separate where you have to demon-pull and angel-glide through to the next carriages. When you get to the front of the train there would be a boss and after killing them the train comes to a crashing halt and you’re in Purgatory. Dante is seeking out help/answers as to why the angels aren’t doing anything about the demon invasion on Earth following straight after the events of DmC.
He realises that angels weren't all exactly powerful enough to defeat Mundus, but to save himself from any trouble from them, he made a deal with them. So Mundus proceeds to enslave humanity, instead of going about destroying it. That way the angels stay off his back, as well as they simply didn't care enough about protecting humanity they cared more about their own divinity and if humans were to be sinful as they were they weren’t deemed worthy of saving. It's not like Mundus was roaming around slaughtering humans on a constant basis, just controlling them through debt and mind control substances etc.
Dante comes to see that this "peace" or agreement was enough for both races, until Dante and Vergil disrupted it. The angels and demons see Dante and Vergil as both responsible for Limbo's destruction, and subsequently the demons flooding into the "human world". Therefore to them the events of what happened in DmC are entirely Dante and Vergil’s fault and the chaotic apocalypse that has followed is down to them.
Furthermore Dante begins to learn more about himself and his race. He sees that Nephilims have the ability to freely enter both worlds, the demon and the angel ones. As well as use both of their powers in either worlds to which he can understand why angels and demons would both fear him and consider him a threat.
But since the end of DmC, Dante has vowed that he will protect humans. He now views himself as humanity’s protector. His brother Vergil is gone, for now, plotting in the background his own maniacal scheme for power to rule supreme over both worlds just like Mundus did before.
Dante would assist Kat in finding other potential followers to aid his cause, and they rally up a team calling themselves The Order Of The Sword (Taking the name from DMC 4) This could introduce Trish as a demon who had a change of heart, like Sparda did. Lady, whose past is like Kat's, except she's half human and half demon because her father sought to becoming a demon and then raped her mother to give birth to her when he was one.
Now you have a small team of fighters against the demons running amok. During all of these events through the game we come to learn that Sparda has broken from his imprisonment and releases his rage and slays a vast majority of his own kind. The only problem is that he has gone completely insane over the years and thinking he has nothing left anymore since Eva's death and the death of his children he sets his heart on destruction and chaos. He's been driven to complete madness and grows to become an unstoppable and unrelenting God of destruction to which he intends to destroy everything in his path and he is purely fuelled by his ruthless anger and wicked aggression.
The angels who have turned a blind eye to humanity since the demons had spent an eternity brainwashing them and tempting them to sin, now turn their sights to humanity in the wake of the demon uprising after Dante finally convinces them he only cares about humans safety and keeping those he loves safe from harm so they no longer see him as a threat and his cause seems just.
Their most elite warriors descend down to attempt to put a stop to the evil and instantly sense an unwelcomed presence, but this is Dante’s brother, Vergil. Having returned from the Underworld with hopes of becoming its new ruler and hopes of recreating humanity in his own image, he is instantly infuriatiated with the presence of angels and a fight between Dante, Vergil and the angels breaks out when Sparda eventually returns and easily dispatches a majority of angels with absolute ease. A concerned Dante manages to convince a hugely conflicted Vergil that if power and control is what he has been desiring then letting their father lay waste to everything would benefit neither of them. Dante wouldn’t have a planet to defend if ultimate destruction is caused the same as Vergil would have nothing to rule over. The two decide to head back to the Order to “gear up” and get assistance from Kat and the other members of the team before they head off to put a stop to their father before he annihilates everything and everyone in his path.
A long ass battle ensues but with the help of Phineas they finally manage to get their father to realise who they are and that they weren’t killed by Mundus which causes Sparda to regret his actions that he took and the darkness that he allowed to consume him. He vows to never return to the human world again and between them they travel back to the underworld to close the portal/veil that conceals the demon world from the human world ultimately ending the demonic invasion. Before they all separate he tries to reason to both brothers to put an end to their fighting and work together to make sure that nothing comes between their family bond. Vergil understands his foolishness and Dante forgives him before they return back from the demon world and the sunsets on a peaceful Earth now completely demon free.