Pretty sure I already said this, but I don't think making an entire game revolve around Vergil would be a good idea. It wouldn't sell according to expectations, since Vergil isn't nearly as popular as we'd like to think he is. Not protagonist material unless he were reimagined to be more like Dante, but that would defeat the whole point of the game. Like somebody here said, making a game with a Dante story as well as a Vergil story would be a good idea. Kind of like DMC4, sans the backtracking. But please do it in DMC style, not DmC. Such a shame to throw away the original series like this, not to mention offensive to the people who grew up with it. A whole unique personality, thrown away and westernized. But this is not about my gripes with DmC
It would be nice to see a DMC game that followed Sparda. I'd say he makes for a very interesting protagonist. The conflict between his demonic side plus relatives and his superiors, and his love for (a) human(s). It's a wonder they never did that story properly, save for the Sparda-like Nero in DMC4. I say lose the gimmicks and create a game that builds on DMC3's strengths. He's Dante and Vergil combined, and it would be interesting to see how things play out in that respect.