Based on the storyline here, and the possible interconnection to modern mythology, is it possible that Dracula could make an appearance as an ally/boss?
The Brood are a hive mind correct? Depending on how it is structured, Dracula could either seek to try and help you destroy the Brood, or attempt to enslave The Brood for his own purposes.
You mentioned potentially making it more like Castlevania, and including Greek Mythology. Lycaon is an ancient Greek figure that tested Zeus by feeding him his own son, to see if Zeus was truly omniscient. When Zeus realized this, he turned Lycaon into a wolf, and murdered his children. You could include Lycaon as an antagonist.
I would mention including such monsters as Medusa, but she is lacking in the werewolf/vampire comparative department.
I hope I have given you guy some ideas. The game is looking really cool!