A few things to note - It was very obviously Vergil that tore off Nero's arm, and I would guess this was for him to retrieve Yamato. He's all corrupted up and still wearing a ragged version of his DMC3 attire, so I'd guess my longtime speculation about what happened to him is correct.
The location here is "Red Grave city". Dante's hometown and the origin of his old alias maybe? I know Dante was supposed (Kind of) to have been taken it from Antonio Redgrave, but who knows. Maybe the city was named after his family, or vice-versa. The more important thing here is how the city's name and the whole "demon tree" thing is very obviously a reference to events from the first Devil May Cry novel. Hell, Nico's role as Nero's mechanic even parallels Nell's role as Dante's weaponsmith in the book. The old guy at the start of the trailer may even be another parallel to the character Grue from the same book. This all leads me to believe that the demon tree spawning this disaster is basically the same kind of thing that was in the book, and that someone close to one of the protagonists is forming it's core and will have to be killed just like, in that case, Jessica. Kyrie maybe? If it was Trish or Lady, it would explain Dante's demeanor in DMC2, should this not take place after that.
As for the main antagonist, I would guess it's either Mundus or Sparda, given that he has the third eye thing going on.
Lastly, there's the matter of the third protagonist seen on the promotional cover art. He's scrawnier than either Nero or Dante, has black hair, and is covered in some kind of weird, demonic looking tattoos. Overall his design doesn't fit Vergil. Currently, my favorite speculation here is that this "mysterious V" is in fact not Vergil, but rather Vergil - Dante's own son, named after his presumed-dead brother. A complete guess, but I like the idea anyway.
