Your forgetting when itsuno said this will be the LAST game following the sons of sparda story, wether dante or Vergil come back or not they won't be a focus. Dante's story is Done. If anything Vergil has more a reason to be in the next game than Dante if you consider fleshing out Nero's story to not be a son of sparda thing1: We still need to see more of Virgil. We HAVE to explore Nero's mother more. It's been dangled on a stick like a carrot in 5. It could be possible with just Nero but it makes more sense to have Vergil in the mix considering how 5 ended. Plus Nero never even knew his mother. Mother aside we also need to see how Vergil has changed after 5s events. This is the first time he's actually stopped a fight of his own accord and cleaned up his own mess.
2: It aren't DMC without Dante.
3: Let's also remember that when a series "removes the main character" the whole series tends to go tits up. It just alienates too many fans. UNLESS the series ends on that note. Maybe move onto a new series with the same theme from the past one. That's kind of what Star Wars did. You need to bring a series to a close FULLY before you can do that though. And DMC has too many unanswered questions with the current main characters for that to happen at this point. Vergil and Nero mainly. Which revolves around Dante because family. Given how Dante feels about family he's not just going to stand back. "Family huh."
Having just Nero would **** off a lot of DMC fans because of the absence of Dante. Not having Virgil in the next game might also be risky. There's everything to gain by having all three (and DMC5 has three characters and works). There's little to gain with only having one. If nothing else you'd have to drag out a series even more and make even more games for each character and that's just more work on the devs then it needs to be (and waiting time for the fans as a result). Capcom have PROVEN they can make it work with multiple characters in the same game. I see no reason to change that. At least not currently. Where we had Dante, V and Nero here we can easily have Dante, Vergil and Nero in the next game.
What can be done in the next game though is what Legacy of Kain did. Where we focus on one character being away from the others in the very same game. Dante and Vergil are in hell currently. But Nero is on the surface. This leaves room to play solo as Nero (like in 4) before/after Dante and Vergil. In fact I consider it likely that this will be the case in the next game. With Dante and Vergil eventually meeting Nero at the end to wrap things up. This would also mean we'd likely be playing as Vergil on his own story arc. What I can't figure out is if Vergil will travel with Dante (like we did in DMC5) of if they're going to split up. I'd like Vergil to be solo for a bit but I'd also like him to travel with Dante. It might start as one then go to the other. Considering Nero is up their on his own right now either way then he has it made. He gets to do his solo things for a bit.