DMC5: Vergil's Downfall (badum-tss) would start out when Vergil and Dante are 8, and would shed light on their mother's death. It would then show what happens to Vergil afterwards and how he becomes the way he was in DMC3. Nothing specific, but off the top of my head it would be based around a 15-16 year old Vergil fighting to free his lover, whom one of Sparda's old enemies has captured (Hell, if you want to add a big twist, it would be the same one that is trying to stop Nero in the plot for Dante's Return, stated above). The demon-lord took her into the demon world to use as a sacrifice for a ritual to undo the seal placed on him by Sparda. In the end, the ritual was successful, and his girl dies. Vergil then defeats him, but before he could kill him, the Demon seals himself back into the Demon World to save his life. Thus, Vergil becomes bitter and power-hungry, quoting himself in DMC3: "Might controls everything. Without it, you cannot protect anything."
Hell, an unlockable after you beat VMD could be that you play an epilogue as Nelo Angelo, fighting through the demons in Mallet Island to get to Dante, because the demons are stupid and only sense Sparda's blood in Nelo so they would attack him.
I came here to chew bubblegum and kick ass, and I'm all out of bubblegum. http://d24w6bsrhbeh9d.cloudfront.net/photo/5062421_700b.jpg
^ I agree with this, since we already know the outcome it would come last. And my pc is down so I don't post as much. I'd also love to see a game about sparda in either series, be it the legendary knight, or how he met Eva, and betrayed mundus in DmC