Tony, thats a very interesting idea, lets explore it.
If we were to face that Demon, the Possessor, would we in essence be fighting Nero or would the Demon exist outside the Vessel that is Nero?
Lets say we fight the Possessor as the Final Boss when we play as Dante, it could essentially be Nero Transformed, and once defeated Nero would be free of the Possession and once more become Human, as we have seen from Movies, i.e 'Exorcist', once the Demon is defeated and expelled, the Human can regain his/her Humanity once more.
But lets say that the Possessor exceeds the being of Nero and can take on a form of his own, maybe we will see Dante and Nero team up and take on the Possessor.
I personally think that if and when the Possessor gains his maximum level of Power, he will become the Final Boss and once defeated will leave Nero without the Arm and as a Human.
But, if we're playing as Nero, what would the outcome there be, would he face the Possessor? Only if the Possessor took on a form seperate from Nero would that occur, or would Nero face Dante in the Final Act, only to lose and be returned to Humanity?