Personally as much as I hope for something like that I'm just gonna assume you'll play as regular gabriel and go through the process of becoming a vampire since it would be the laziest way of doing it.
Implementing an entirely new combat system for multiple weapons that doesn't look childish compared to the frankly insanely complex combat system the game already has would warrant a new game.
As an unrelated note, something has been pestering me since last night.
Kojima.
I know, his involvement is limited, but at this point, CL:LoS is basically Metal Gear Solid 3.
I have visions now.
Castlevania: Simon's Revenge.
Simon gets arrested by the Inquisition and sent on a suicide mission to eliminate a traitorous Crusader who's assaulted a castle in Germany and took a few important nobles hostage. His only clue is that the guy's name is Liq... Liam Belmont.
I can already see it. "Brother! You've been fooled! All I want is our ancestor's body! We were magically cloned from him... Les Enfants Belmonts!"

Stuff of nightmares.