This will sound contradictory coming from a fan of Vergil, but I am not sure he would support a full game as we know him without being altered. When I say 'support', I mean, hold up in the fans' eyes, story-wise because he is essentially a foil of Dante. Unless he's given a compliment of great characters as well like there was in DMC3, all of which are interesting in their own ways, would a game with Vergil as protagonist not run the risk of being too dry and devoid of emotion? A quest for power-ups is what most games entail indirectly, but it isn't really that interesting an idea in itself. One reason I began loving the series and the character of Dante is because he cares and doesn't care at the same time, but he cares about the right things and he has a vulnerability. So for Vergil to work as main character, I'm guessing they're going to have to change him so that he cares about something a bit more than just POWAR and that he has some kind of weakness. As we know him now, he doesn't have a weakness so to speak, except the weakness of thinking he has no weaknesses... and although he appears to care about Dante, he doesn't care enough.
If the Nero/Vergil thing were used, then they could go into some backstory there and give Vergil good, powerful, emotional reasons to do things. But a lot of fans are NOT keen on the idea that Vergil is Nero's father at all. So who knows what they could use while still maintaining a recognisable 'Vergil'. The only other thing I could think of was that Vergil had some adventures once corrupted, but again he'd need a reason for us to care about what he is doing. Can they give him one without screwing up who Vergil 'is'?