Wow, alright, here goes XD
Nero might not have his own Devil Trigger yet. As you can see in DMC4, he has a combination of his own DT and Vergil's DT floating behind him. Note the halo on the blue floating demon behind him. Vergil or Nero Angelo didn't have that, while Nero does in concept art (my avatar). You can also see that Nero was, at some point, meant to have a full Devil Trigger like Dante. I think Nero is relying on the Yamato too much, or somehow needs his own weapon like Rebellion or Yamato to awaken his powers. Rebellion seemed to awaken Dante's DT, so the same could happen with Nero. I suppose DMC5 could focus on him being injured with something and getting his DT. Maybe you'd have to battle Dante and defeat him as a sort of 'the student surpasses the master' type of story.
That, or they could show how Sparda's rebellion went and what awesome demons he must've encountered. Perhaps a cameo from Berial
Naah, guess not. I would remove his monocle, I think. It just seems silly to play as a posh monocle-wearing character.
Or perhaps play as Vergil to see what he did before DMC3. Or perhaps his soul survived after he was defeated as Nero Angelo and is fighting his way through some kind of hellish place.
I don't know, there's so much to imagine. I'm really not awake enough to think of more ideas
I wrote that Nero stuff in the thread
Nero's Origins.