Maybe Nero's last boss could be this boss I came up with YEARS ago, right after I beat Dante's disc in DMC 2. You know the Despair Embodied? Imagine it, but bluish white, and with the wings already broken. Also, it's first form is stuck in male mode, and it's second form is the female mode, and she has wings, with the final form having the moves of both.
Chaos Incarnate: Ver. 1 (Man Form)
Chaos, put into a malleable form for the evil energy that binds it. Capable of vicious combos with whips, swords, energy, and even punches and kicks. Although it cannot fly like the Despair Embodied, it doesn't need to. It can teleport instead. Your attacks at first will deal no damage. That is because you must buster it 10 times, so it exposes it's core. Then, you come in and smash it with all you've got. After around 10 seconds, he'll recover.
Buster: When he exposes his core, buster him and Nero will grab him by the head and stab him in the gut with Yamato, causing a white, luminescent substance to spew forth. Nero then pulls Yamato out and throws him up in the air, then he jumps up and uses the devil bringer to punch him back down.
Cutscene
CI: You are... Stronger than the one we intended to bring down here... Who are you?
N: Does it really matter? All I know is that I'm gonna be sending your blue *** further down!
CI: Heh... hahahahaha!
CI transforms into his/her next form.
CI: Then why don't you try, bluecoat?
Chaos Incarnate: Ver. 2 (Woman Form)
Stronger than before, and she now has the ablity to fly. She can summon ethereal whips that are dissipated in one hit, but can cause tremendous damage. In addition to the power of flight, she is also faster, and her wing-slashing combos are devastating.
Buster: Nero grabs the CI by the wing and slams her into the ground, which makes a whip spawn. He then grabs the whip and throws her, but then he wraps the whip around her and reels her back in, then he grabs her by the leg and throws her for real.
Cutscene
CI stands, panting, out of breath. She then holds her head and throws it back, screaming. A bright white light appears, and when the light fades, there's a dark knight standing. All of his body, except for his angel and demon wings, are covered in armor. He looks classy. He even has that old fashioned T-visor. His sword is gigantic. It's even bigger than he is! He carries a large, spiked shield in the other hand. His voice is deep, menacing, and no doubt evil.
CI: We've underestimated you. But no longer! This shall be your graveyard, bluecoat!
N: Oh just SHUT UP and FIGHT already!
Chaos Incarnate: Dark Knight of Hell
He's not messing around this time. His dark sword creates wave of dark energy whenever he slashes, and his dark shield repels all attack. He slowly marches towards you, slashing all the while. To destroy his shield, you must buster the rocks around the arena. After bustering three of them, his shield will break, and you can attack.
Buster: Nero grabs the Dark Knights sword and struggles a bit, then he rips it out of his grip and slashes him an uncountable number of times with both the sword and Yamato. The Dark Knight then raises his shield, but Nero grabs it and throws it at him like a giant bladed frisbee, slicing through him. Nero then throws the Knights sword at him, but he catches it and the battle resumes.
cutscene
CI is standing, looking defeated. Parts of his armor are broken, and some are missing altogether.
CI: This is impossible! We never predicted that you OR the other could have defeated us! IMPOSSIBLE!
CI rushes at Nero, but Nero slices through him with the Yamato, and begins to sheath it.
CI: Ngh... What... Are you?
N: I'm Nero.
Nero sheaths the sword and CI fall over, cut in half vertically.
N: And you got blood on my sword.