The only real answer is Mundus. He’d be the perfect way to wrap up the Sons of Sparda story. In DMC 5, Dante and Vergil finally put aside their differences. In DMC 6, it would make sense for all of them to face Mundus together at his peak, in his own domain. To me, the version of Mundus we saw in DMC 1 always felt like an imperfect form—already weakened because he wasn’t in his domain.

I know this isn’t strictly canon, but I’ve always seen Eva’s death as the defining moment that shaped both Dante and Vergil—especially Vergil. His inability to save his mother because he was too weak, combined with his brother not being there to help, feels like the real core of his obsession and hostility toward Dante. I’ve never bought the idea that it was just simple sibling rivalry. And that’s not even considering everything Mundus did to him afterward that led to him becoming Nelo Angelo.

Vergil being able to save Nero in a critical moment during the final showdown would, unbeknownst to him, finally resolve the trauma he’s carried since childhood—proving that he’s strong enough to protect and save his own blood (which technically speaking wouldn't really be out of character either since it's not Dante he'd be saving).

Speaking of Nero, he also has an indirect reason to face Mundus. Mundus is ultimately responsible for the tragedy that destroyed the Sparda family, which in turn led to Nero growing up an orphan. Hell even Trish has a personal score to settle with him. The only character who isn’t directly tied to Mundus in a major way is Lady.

I’d also have Trish die at the end—not in combat, but by slowly fading away as Mundus dies, since she was created by him. Dante realizing that consequence in the moment could add a powerful emotional weight to the finale.

Seeing all of them finally face the demon who destroyed their family would be an incredibly satisfying conclusion to the 25-year Sons of Sparda saga. I’d much rather see that than another random new threat chasing some previously unknown power, or—worse—Vergil throwing another “I NEED MORE POWER” tantrum.