I know everyone is sick of the comparison, but ever since Demon's Souls I've considered the Souls games and especially Bloodborne to be spiritual successors to Castlevania. Obviously they have many differences, but imagine a more fast-paced and focused Bloodborne with some rocking tunes and you basically have a modern Castlevania.
Either that or just 2D ones, but I'm kinda done with the Metroidvania style Castlevania's tbh, they've never come close to being as good as Metroid with its mediocre level-design, but go back to Castlevania's glorydays (I, III, IV, Rondo) and you could have a solid seller provided the quality is there.
But ultimately: just let it go please, it's never gonna be relevant again so at least let it die with some dignity intact and don't ever let Mercury Steam tarnish it any further than they already have.
What would be gained by doing this?