The music played when Nero is making his way to the Opera House is more associated with Kyrie than Nero himself. They replay a bit of it in the scene where he and Kyrie are absorbed into the Savior and Nero tries to reach for her and she disintegrates at his touch.
But for a theme song for Nero? Nah, it wouldn't work. As Lerrion said, repetitive music in all the installments smacks of lack of effort on the producing team's side.
Although I have to say, I disagree with this.
That's a contradicting suggestion in the face of the general consensus of all DMC fans. A lot of people have complained that DMC4 isn't gothic enough, not dark enough, and the music in DMC4 is some light metal mixed with, I do believe they call it, love metal (the ballad style at the end of the Surrender song) Plus, the background music when moving through missions is a bit more creepy with the sweet, twisted and off beat cymbals, whereas in DMC3 you often just hear a monotonous tone and children laughing every once in a while.
My point is, DMC4 IS completely different to DMC3, in style, themes, story and music, and it wasn't such a great big hit with fans because they wanted something more DMC3, or DMC1. If the next game they bring out is like Bayonetta's themes and music, the fans are likely to complain about it as well.
I'm not picking a fight or anything, btw, I'm just pointing out that bringing something different to the table won't necessarily make for a better game, because a lot of people are looking down on DMC4 FOR being different.