My theory:
Nero and Dante are mutual protagonist/antagonists and both will be playable characters.
Some points:
Nero: highly probable or confirmed points
Given: Nero is a good guy.
Given: Nero is part of this Order of Knights that serves/worships the legend of Sparda.
Given: Nero and Kyrie have a relationship of some sort.
Given: Nero really doesn't like Dante, at least at one point.
Given: Nero tries to kill Dante.
Given: Nero has something that belongs to Dante.
Unknown: We do not know if the Knighs are good guys or bad guys.
Unknown: We do not know if there is a more powerful third party pulling strings.
Unknown: We do not know Nero's biological heritage.
Unknown: We do not know the source of his weird arm or where he got his sword.
Dante: highly probable or confirmed points
Given: Dante is a good guy.
Given: Dante kills demons.
Given: Dante has a lot of toys/demon artifacts
Given: Dante initiates an attack on Nero and the Knights in a temple setting.
Given: Dante has killed humans in other DMC installments*
Given: Dante kills some Knights.
Given: Dante does not want to kill Nero**
Given: Dante is still his cocky self
Unknown: What did Nero take from Dante
General game points:
Given: Basic monsters everywhere.
Given: Nero is constantly killing basic monsters.
Given: Dante will be constantly killing basic monsters.
Given: At least one fricking huge demon must be dealt with.
Unknown: The Knights do not necessarily know who Dante is.
Unknown: The source of the basic monsters everywhere.
Unknown: The significance of the rediculously big fire demon, is he a plot important villain (Mundus DMC1), or just a really big nuissance like the building dude in DMC2?
*DMC2, main villain was human for most of the game, Dante faught and defeated him first as a human.
**Will let Nero go if he returns what he took from Dante
Now on to the meat of my theory.
I believe there is a third player aside from Nero and Dante. This player is the true villain. Nero is a pawn. Dante is a Devil Hunter. Over the course of the game Nero will grow into an extremely powerful Hunter himself as he learns what is going on.
I think Nero and the Knights took something from Dante, most likely the amulet and probably also a Devil Arm (the right gauntlet of Ifrit?). They did this under the command of the higher villain and (Nero at least) were not aware their actions would open the gates of hell.
Dante comes after Nero to retrieve what was stolen. Through the course of his troubles Dante learns of Nero's power and more of his role in this whole debacle. Dante may also learn of the role of the Knights.
In the temple fight scene Dante butchers the Knights. These Knights were either truly evil or Dante acted in self defense. I personally favor the former.
In a later part of the temple fight scene Dante offers to let Nero go if he hands over whatever it was he took. Dante dodges Nero's attacks and is plainly trying only to be evasive and not kill the poor dupe.
I suspect the temple fight scene ends with the gates of hell opening and the very large demon coming through. At this point, I suspect Nero realizes Dante is an allie and not an enemy. At this point also, the main villain is revealed to both pro/antagonists. The good guys team up and through two different viewpoints kill the villain.
On the origin's of Nero: I would not be surprised if he is a son of either Virgil or Dante and I would not be surprised if Kyrie is his mother. Nero bears a striking resemblance to the Sparda twins (platinum white hair, same height and build, same fighting style) and has the power to control devil arms. If you'll recall from DMC1 and DMC3, Devil Arms must be defeated by their possesor before they can be controlled. Clearly Nero does possess some demonic qualities.
That's my theory.