I think Nero should get a chance for the spotlight. The whole idea of DMC4 was that Sparda sort of returned, in the form of Nero. If he turned out to be an experiment created by combining Dante and Vergil's blood, or by genetically modifying a human with Sparda's power, think of the possibilities. Not only would there be no need for a Sparda game, but the circle could also be completed. The legacy of Sparda gets to be continued, since Dante doesn't seem interested in having children. Nero IS interested in having kids, most likely. So this way, we get a close to the DMC series.
Just make Nero a little more badass, give him more weapons and more combo possibilities. Give Dante a few missions again, or have Nero rescue Dante after DMC2. Then, at the end of the game, have Nero and Dante truly fight together as equals. No more 'Dante to the rescue' or 'Dante is 100 x more powerful than Nero' - no, just make Nero grow as a character. And hopefully, give him a full Devil Trigger too - no more 'blue, floating Vergil' bullshite.
YES! Like InfernalOverkill said, make us able to choose Nero or Dante for each mission. Maybe even give us the chance to customize them in terms of appearance. I know the DMC series, strictly speaking, is not a series. It's just a bunch of games, each with their own plots. But Capcom has the chance to end the DMC franchise with a bang - one that does justice to DMC1 and DMC3. If the other games don't form a continuity, then at least make DMC4 to DMC5 a coherent story. Close off the demon world for good, or find a way to regulate it. Give DMC (especially Nero) a conclusion.