I think Zobek will be the final boss, as I'm sure his "deal" is just another deception, Gabriel will kill satan and then Zobek will acquire his powers or something, then we have to kill zobek. And for emotional punch, the last boss is Dracul going back in time to kill Gabriel Belmont at this prime so making the most epic ever final battle and then ereasing his timeline from the existence (remember that MS said that EVEN if THEY work on a next gen Castlevania themselves Dracula and cast would be different) so killing himself in the proccess and avoiding all in the process.
Then we'll learn that in reality God's will can't come into fruitition without a Dracul and then we see glimpses of Leon Belmont or Mathias or whatever, reconciling so the new series with the old one, and learning that, just like Alucard words said, there will always be a Dark lord.