Some great ideas in here. Wish I had seen this thread when I shared my Frankenstein-based CV idea in that other thread. Ah well. I'll post it here too but I'll stick it in a spoiler tag so it doesn't take up much space.
I always had this idea for a Castlevania game that borrowed more from the Frankenstein mythos than Dracula.
Basic idea is, you play as the original Frankenstein Monster. Someone is following in Victor Frankenstein's footsteps and creating an army of monsters, and it's up to you to stop him. You must venture into the mysterious castle and kill his legion of strange and horrible creatures before they can be unleashed on the world. You can probably guess just what castle this is, of course, even though a mad scientist has taken up shop in it.
As the original Frankenstein Monster your primary weapon is a chain, used much like a whip in fact. Your secondary attacks are all based on your ability to harness lightning, depleting an energy meter rather than a magic meter, and these can also be used to solve some puzzles early on. As things get more difficult, a mechanic is introduced which is inspired by a certain famous blue bomber: with every boss you kill, you acquire a body part you can attach to yourself. With each new limb that you swap out with your old limbs, you enhance your attributes, learn new skills, and/or obtain new means of attacking your enemies. For instance, a new set of legs could allow you to jump higher. A new arm could mean a weapon with a longer reach. Some limbs may aid in solving puzzles while others may make things easier when dealing with particularly nasty enemies.
The catch is, you can't switch between them willy nilly. Throughout the castle are surgical stations where you make the necessary alterations. You also have a limit to how many parts you can use at a time. You've only got two arms and two legs, after all, and going much farther than that would require more extensive surgeries. Because of this you'll need to be clever when it comes to which parts you use as you travel through the castle. This will also make a difference between how you play the game and how other people play it.
At the end of the game, you confront the mad genius behind all the monsters, one Doctor Septimus Pretorius. However, you soon learn that his ultimate goal is to make a monster much more diabolical than any other you've faced: he has obtained the heart of Dracula and not only has he built a new body to house it, he has tricked you into using your lightning powers to bring it to life. So the final boss is Dracula after all, resurrected artificially through mad science instead of magic, and the result is no less than an abomination.
After completing the game, you unlock an option to play the game again as the Bride of Frankenstein, of course, who has a unique play style all her own and possibly her own storyline.
The thing about this idea is that it would be very easy to rework into an entirely new IP that retains something of the Castlevania flavor, with its strong emphasis on classic monster lore and gothic horror weirdness. Still, while it would be a bit of a departure I reckon the general concept could work better within the established canon than some other ideas which were actually carried out. Anything's better than the big ol' stinking pile of nothing we have right now.
What say y'all?