You are probably right. I doubt Vergil will become evil or be broken. But I think Dante will form a different view than Vergil on what could be anything relating to demons, and they will part ways, both combating demons with their own philosophy.
I think this is what will lead Dante to become the owner of Devil May Cry.
how about a boss that has a backstory with sparda? Somewhere along the line of someone who could've rebelled with sparda but instead chose not to and was the reason why sparda fell after sparda's rebellion. That would be pretty good if you ask me even if it sounds a little cliche.