Oh, and plus, having a game with Vergil in Hell would give them the oppurtunity to let us see how demons there would react to the presence of a son of Sparda in their own territory. They hate Sparda and his legacy's guts, so that would be cool to see.
Also, the most interesting trait about Vergil's character was his inner struggle between what he his - a human/demon halfbreed - and what he wants to be - a full, powerful demon. I see a game with Vergil in Hell having that coming even more into play. If they chose to use the approach I mentioned before, maybe they could play with the fact that he is rejected by his own kind, coupled with the fact that his human side is also rejected, by himself. Play with his identity crisis, so to speak. Exploring his character in that sense.
But a Sparda game set during his rebellion, I've always said it and I'll keep saying it, would also kick all kind of ass. There are so many possibilities to pick.