I'm just throwing this into the ballpark...
Maybe they're adding Nero because they CAN'T do anymore with Dante. Dante already pawned the two most powerful demons in existence. The King of Demons-Mundus and his brother-Vergil THREE times over. And if you add the book he pawned Mundus a second time. Going with with what another forum poster said earlier, the first and third game completely tower over any exploit he does in the future. He's already THE most powerful man-demon in the universe. The only way they could make this work for Dante more without it being cheesy is reviving Mundus early and there's a period of three games where there's a literal WAR between the humans and demons and Dante has to step up like his father did. Maybe even get Vergil back in somehow to help in the cause. OR play a Sparda and do the same thing. It would be interesting to see the series through Sparda's eyes and actually get to to know him and Eva. To add to new playing experience, Sparda most-likely fought like a modified version of Vergil with Yamato and Sparda and all.
Actually, I scoffed for him when I watched the TV series. The final demon... PAWNED. I wasn't even worried. Dante is TOO strong, he only gets nervous around Mundus in the human world, and he still kicked his ass. Only Vergil can hold a candle to him.
Now if for some reason Dante ended up fighting God... THAT would be interesting....

Iron Chef...