Umm... Dante was supposed to be in Soul Calibur III, but for whatever reason he was cut from the game. Anyway, I can see Dante and Vergil, both DMC and DmC versions, in the Soul Calibur series, however, I can't see Nero since his Devil Bringer might break the game. Think about it, a ghostly arm capable of growing in size and can grab enemies so Nero can pull himself towards or to him. At least Ophion is more of a grapple hook that people can deflect, but a demonic arm? The stupid thing doesn't feel pain and could just grasp the blades of enemies and rip them away from them. Even if you deflect it, Nero could just make a larger arm and beat people to dirty pulps.
That's if developers don't make Nero stupid like making Nightmare stupid. For whatever reason, we never see Nightmare just use his claw to break weapons, beat people to death, or as a shield. Oh, don't forget that Ezio could just use smoke bombs or that he and Cervantes have guns. Just headshot everyone. Then again, this is a game where there are sentient weapons, people with ridiculous weapons and outfits, and immortals.
Nero could be the Starkiller or Yoda of Soul Calibur. I think Starkiller was considered a little overpowered since he moved and attacked quickly and brutally. As for Yoda... The little green sage is up to their knees and he can avoid so many attacks just by standing. Freaking green midget.