The press release says that this boss is notorious and that every Castlevania fan is familiar with him.
So, the two guys that appear in (likely) every game are: Dracula and Death.
We could exclude Dracula, because it seems Gabriel becomes Dracula in the future. However, he could fight Dracula and claim his power.
Death is more likely the good one IMO: remember the trailer (or Cornell's speech, I think) "A true warrior who can claim the power of the Lords of Shadow as his own" -> Gabriel got Cornell's speed, Carmilla's flying, but nothing from Zobek/Death-guy. Thanks to all these powers reunited, Gabriel could become Dracula. Plus, in the ending secret future scene, Gabriel asks Zobek where he had been all this time: it could be a hint to the fact Gabriel or Zobek defeated the other one (and never meet until modern times).
Other candidates could be:
- Slogra: he has been a famous boss/enemy since Super Castlevania IV (which is the "bible" of Mr. Cox) and his corpse appears in the end -> Gabriel could have kill him and stay in the chapel for centuries after that.
- A Belmont: Yeah, Belmonts appear in every game (likely) and if Gabriel is possessed by darkness, he could even fight a "good" guy. Simon or Trevor cold be fine. Or not.
- A Cronqvist: Gabriel is a Cronqvist, rejected from his family (according to the site or a guide I think). And in LoI, Mathias Cronqvist becomes Dracula. The two stories might be connected. Or not.
- Galamoth: Plays with time, and is the enemy of everyone (even Dracula, Aeon, Death, etc.). Why not?
It would be a shame they put a second Satan or another Titan fight, but I think Zobek/Death/Lord of Necromancers will be the one Gabriel will confront.