A fully open world game with town and villages akin to Red Dead Redemption, but instead of desert it would have thick forests, mountainous area, caverns, swamps, ruins, cemeteries basically all your Castlevania type areas. All with area specific roaming monsters. Different castles, mansions, and larger ruins would serve as somewhat self contained levels. Castlevania would just be ruins near the center until it is ressurected and becomes it's own self contained level, or game if you prefer. The towns and villages would be full of people you could talk to like in Dragon's Age or Skyrim, giving you side missions and quests. The series' lore would be spread out through various books and villagers' stories like in skyrim.
The story would be a retelling of the evens in the 1800's. Players could choose between Reignhart Morris or Shanoa. Whose secret histories are revealed through their stories. Reignhart would receive the vampire killer, nor Shanoa Dominus until near the end of the game. Their playstyles would be very different, but with a lot of variety.
Reignhart's combat would be combo based, but not so exagerated as something like God of War or Devil May Cry. I would want a lot of weapons available, each with their own uniqe combo styles, like in Curse of Darkness but more modern. Subweapons would factor in heavily as long range attackers, and a rifle subweapon would be a must.
Shanoa would be more similar to her glyph style in OoE but in 3D wih more variation in physical and more spell/long ranged type attacks, with lots of morphs of course.
Platforming would also be heavily implemented into all the self contained level areas ranging from LoS/SoTC type scaling to more assassin's creed type free running, depending on the area.
Beating the game would unlock characters like Carrie Fernandez, Henry Oldrey, Albus, Cornell, Nathan Graves... as many as they could fit in, expanding on different play styles of the primary two characters. Boss rush of course.
This thing has been kicking around in my head for years. I even made a map using the OoE map and icons.