If 3D:
Stamina based combat - no combos, Castlevania should be about precision and knowing the correct time to attack IMO.
Souls like exploration with small cities/locations to visit- Think Castlevania 2. Not so huge to be walking aimlessly but with enough space to explore, with tight level design.
Julius Belmont as the protagonist in the Demon Castle War. When the Belmont clan finally defeated Dracula once and for all.
If 2D:
Classicvania, a whip, and different paths, like Castlevania 3. Perhaps with some skills and new itens but the design needs to be careful to not mess with the formula too much. It should be about the action per se, not grind or loot.