They have never felt Vergil was strong enough to have his own standalone game , only about him
How do you know that won't happen in the next game? It's the perfect opportunity to do so.
They wrote Dante into a corner as mundus is sealed and all the demon kings etc are defeated and gone
They also wrote Dante appearing to be the bad guy in 4 which brought him back. And in 3 with his brother. You can write up anything to bring back past characters. Provided the plot makes enough sense. It happens all the time in games. Of course they kind of mucked up the later half of 4, but that's a game design flaw which lead to compromising the story (I imagine a good chunk of 4s later half got scrapped and was replaced with the same boss fights). In 5 he's back because of his brother. Again, yes, but in a very different way then from 3. They
wrote him back in. They also wrote Vergil back in, despite him being "written into a corner". If that didn't happen DMC5 wouldn't exist in the first place. As for demon kings, there's always another demon trying to claim the crown. And it doesn't have to be about demon kings either. A "king" isn't always the "biggest threat". In DMC3 it's even a human. My point is you can always "write yourself out of a corner".
Nero has potential growth still and an enemy like mundus would be a real test for him
We don't need a game focused on JUST Nero for this to happen. It's perfectly possible for it to happen with Dante and Vergil as well. While being "away from them" in the same game. As I stated before. We also ended DMC5 AS Nero, which means Nero had the spotlight at the end of the game. Vergil WAS his test. He stood up to his own dad (who gives even Dante a spanking of all people) and fought for his acceptance going "Find another way then killing each other". Nero in the end of DMC5 was the most grown up one of all. Where Dante and Vergil were acting like stubborn children (wait, did Nero... spank his own dad?) You could even say Dante and Vergil are the ones that have grown up. Because of Nero. Nero might be an immature ass at times but he gets the good things done. But that can be said for Dante too. You'd be surprised how often "being an ass" works. Being naive (or ignorant) is another matter. Is Nero naive? What does he need to "learn"? How does he need to grow and why? What if Nero is still acting the same way in his later years because it's the only approach that works without people killing each other? What if being "too realistic" is the problem in the first place and why Dante and Vergil tried to kill each other?
Also he is young and still becoming established
So was Anakin in Star Wars. We still had Obi Won and Yoda. Anakin even has his own apprentice in a spin off movie. This is just only proving that Nero needs
guidance. He JUST found out he has a farther. Nero having a talk with Vergil about how to avoid "past mistakes" would be nice. In a serious, none mushy manner. At the same time Nero also needs to be able to live his own life and do his own thing. But a DMC game focused on just him alone isn't the way to do it. From a business standpoint you stand to lose customers due to not having Dante. I'm one for taking risks but that move is too risky. I don't want Capcom to play it too safe either, but 5 took a fair few risks and pulled things off. But that's because they made very sure they'd keep all the key characters involved. Once you take characters away it becomes more of a problem. People are already unhappy they can't play as Mary and Trish (which is understandable as they're SIDEKICKS. Dante and Vergil are not sidekicks though). Start taking away main characters and fans are going to flip tables. People are just too attached to them at this point.
Fan service aside Dante and Vergil are getting older and it's obvious from 4 onwards that they are trying to pass the torch onto Nero
And as old characters people have come to like them and want them to stick around. Despite or even because of their age. This isn't Final Fantasy where you can just replace the main characters willy nilly. Just because you want to "move on" from them doesn't mean others do. There needs to be a compromise. And having Nero on his own in the same game with Dante and Vergil away from him and having them meet up at the end sounds like a good compromise. A topic you have completely ignored and one I am now currently pushing. Give me one good reason why we can't do that. You get what you want, people that like Dante and Vergil get what they want. It's win win.