A game set in the world of Akuma no Chi that expands and continues the story. The plot and dialogue would be written by author Ichiro Tezuka and the artwork by Masaki Takahashi. The game would be designed similar to Demon's Souls and set in a seemless open world which would be medieval Wallachia. The game would star the four Belmont characters from the story: Simon, Ray, Lias, and Civil, making the narrative switch between their view points. It would be more dark than usual in order to fit a vampire story. Not purely gritty and dark like Lords but also elegant. Being based on a novel, the game would be more plot heavy than your average Castlevania game.
Other things I would do: introduce Kid Dracula as a mascot character who often makes cameos, a Castlevania RPG, try to get a solid fighting game spin-off series off the ground, and a game with a more stylized art direction similar to Ghostrick Alucard or the Vampire Karin anime.