Hm, how about rebooting Beowulf?
He would still hold this huuuuge grudge against Sparda and be on a hunt for Dante (and possiblely Vergil) and could be used to elaborate on Sparda's story a little more, instead of letting Phineas do all the story telling
As for Nero and his crew, I really like the idea of them being genetic experiments, as darkslayer13 suggested. But instead of rebooting the whole Order of the Sword, why not make them part of a side branch of The Order, from another part of the world? That way The Order could be explored a bit more, the Nero-Vergil connection would still be there somehow, but better explained and more logical, and looking at conceptart, they originally wanted to incorporate experiments on demons, so they could take up that idea again :>
Or what BLACKSWIPE wrote, seriously, that was such a great and well developed idea!
Guuh, I'd love to see Nevan again! Love the "Lilith's sister" take on her, that way we'd also get a direct connection to the first game which would make things more complex, I guess.