Belmont as a main character.
Dracula as the main antagonist, no more origin story bs. Just make him the guy you fight at the end, plain and simple.
Traditional platforming.
Make it less llike God of War, or perferably nothing like God of War.
Make it challenging, yet fair.
Have more traditional Castlevania music, Get Yamane back if possible or someone of equal talent.
Have it set in a castle full of monsters from B horror movies and various mythologies.
Make a new timeline or set it in the old one, doesn't matter either way.
And finally:
Make it 2D, not 2.5D, not 2 half and 1/4 quarters, 2motherfuckinD.
BAM, there's you're Castlevania.
P.S: make it a Classicvania like Adventure Rebirth, Give the Metroidvania format a rest.