Here is my theory and all revolves around this one picture.
I think this drawing could be a symbolic foreshadowing of a Dante VS Demon Dante (of some sorts).
First lets examine the image.
1) It shows 2 guys who look exactly like Dante (this one) and Dante in his demon form (DT) so its highly about Dante.
2) The 2 guys appears to be fused together or is fusing together to become 1 being (hinting at Dante fully mastering or completing his demon blood).
3) The biggest of them all, it shows 2 guys who look the same and appears to be using the same weapon on each side fighting each other.....like an inner war.
My theory is that for Dante to fully master his demon powers he was must defeat his demon side like literally face his own demons. So it works like this. Remember the beginning of the new tgs trailer that it showed Dante going DT in what appears to be some white space almost as if he was in some surreal dimension (his inner realm) that I think comes out when he enters DT (reason why when DT activates the area becomes gray and it looks like a new dimension) so Dante enters this dimension you can say to face defeat his demon side to overcome either some rage or his demon powers to become a full devil. So I think him going DT in that trailer is a scene of him fully mastering his demon powers after conquering his demon side.
Also the E3 trailer showed him going DT and it looks like he was being possessed.......demon side go figure. Anyway, I think that when Dante goes DT he isn't all that self aware or views it like how we see it. Its sorta like how in NARUTO, at the beginning Naruto would activate his own demon fox powers and didn't know it or didn't notice he just got angry and got a major adrenaline and power boost....that is how he saw it. Same goes for Nero, I doubt he actually knows that there is a hovering spirit behind him he just knows going DT makes him stronger like I think this Dante when he goes DT doesn't know his hair just turned white and his jacket turns red and maybe he isn't really paying attention or isn't fully himself when he goes DT...almost as if demon side has taken somewhat control over him like those instances when Naruto would go fox mode (he'll know of his actions but doesn't know his inner demon is actually mildly possessing him). So when Dante goes DT, his demon side takes over somewhat allowing for Dante to not judge his actions or what just happened he just knows that he has an inner power he can tap into and cause a lot of crazy stuff to happens (this also ties into when Dante goes DT his HUB face changes from regular Dante to a twisted evil demonic Dante face....almost like a split personality). I think Dante could've always be able to go DT or at least before the game's story starts. Yeah I think you have DT on the spot (like DMC2) or at least at the near beginning (like DMC1).
How does this apply to Vergil he either A) went through this process or B) didn't have to since he appears to have white at a very young so he must've inherited more of his father's genes allowing him to be born with white or inherit it earlier making him genetically more superior to Dante...like those stories of one twin being better than the other twin through genetic dominance.
I'm hoping this is true so it can lead to a Demon Dante boss fight just so it can add more boss fights to the game (I see only 3 boss fights and 7 possible bosses).
However, if that is not the case then the picture could hint at a battle between Dante & Vergil.