I had another idea... You know Vergil can teleport? Well I was thinking that, when you are playing through Dante or Nero's path, you can call for Vergil's assistance for a short period of time.
Like, Dante and Nero make a sorta taunt like "Come on"... As to which Vergil appears to help out, and you can even switch to him for that time. And you can also perform moves together... So the two characters actually interact with one another to help fight.