Here's a small analysis...
Dante - This character has so many ways to be used in the storyline, and still Capcom hasn't bothered to integrate a more serious approach. After all Dante has been through, in DMC4 - which chronologically corresponds to be after DMC1 - his humorous and arrogant nature has still the upper color in his personality. One would jump out of his seat and shout 'What about Devil May Cry 2?'. Here I say that Capcom is getting away with a justification such as setting Devil May Cry 2 so far in the future, making Dante, a basically used character in the series. Now, It's time to end It with his future in the franchise - unknown as we speak.
The Son of Sparda has served most of his purpose, and although being the trademark of Devil May Cry, he already did a step backwards in Devil May Cry 4 when Nero was introduced. I guess Capcom is testing the reaction of fans towards such approach to them. Probably Devil May Cry 5 won't have Dante at all. But in the end, when you'll read this, the future paragraph is just speculation, so don't get hopes up or down.
Vergil - On the contrary of most fan speculation or so-called, 'confirmed facts', I believe he's the key to revive the series' main plot - that is what triggered him and Dante to shape their personalities - the death of their mother and the disappearance of Sparda - the latter not being of such a big influence.
Looking at the name Vergil and next to It, 'confirmed facts', we understand that nothing is really confirmed about him, except for what is written, shown and said in Devil May Cry 3. We all know the huge contradictions with Devil May Cry 1 It has. If I were you (generally-speaking), I wouldn't literally take 100% of what's said and shown in the cutscenes of the original game as canon. For as much as I know, Vergil could be trapped in the Underworld or just a silent observer of the events.
I count a lot on his return and I wholeheartedly agree on having him as the one who would narrate the other side of the coin for who knows how many years.
Nero - At first glance, had a repulsive opinion on him. Thought he'd be just something Capcom had invented in order to clone a DMC3-ish Dante with a mix of Vergil's and cherry Sparda on top - which he really is right now - get the contradiction?! Doing some short readings and most importantly, after playing with him through the entire game, made my opinion change from black to white - huge contrast.
All fans went nuts after the Vergil theory, and as of late, most have accepted It after a so-called member of the Capcom Unity staff revealed info about the Alpha stages of the Devil May Cry 4 game. But, If you really want to have a clear and personal judgment on who Nero is, you will probably have to take a look at the colors which surround him - red and blue - colors which united make the purplish color which Sparda is best-known of. Let alone the hints of Sanctus, his personality (Dante+Vergil=Sparda), and last but not least, him having a love interest, reminiscent of Sparda's love for Eva. I foresee Nero as the one who will lead the Devil-Hunting business, in his own way and at his own location - at Fortuna. I don't see him as Dante's direct successor, but more to as a fresh addition and a plus. It would be interesting to see an actual preview of the three Sons of Sparda. That's enough for Nero.
Sparda - The Legendary Dark Knight, Savior and Guardian of the Human World. 'There's much confusion surrounding him.' - were the words of Dante. The most mysterious figure in the series. Where is he? Is he trapped in the Underworld with Vergil, or does he have a plan to come back. Would he really hide all these years after his beloved one's death and their sons' hard childhood passing in front of him just like that, or did he have a really good reason to do so. I can ask literally 3,564,123 questions about Sparda. So, by saying that, I'll skip to the future part. It would be an interesting twist of events have him appear. I dunno how it will be made or will It be any good. Watching how the series went downfall in terms of storyline with Devil May Cry 4, I can see him used as a Desperation Devil Trigger, only this time, to save the storyline for being totally torn apart.