The one thing I am sure of is that stakes should be a sub-weapon. I really don't understand why they haven't been in the series much.
The stake was an inferior version to the sword/dagger, and it was only used in Simon's quest so there was no real need for it. Also the mythos of the oak stake is kind of lost since the Belmonts use the VK.
Jumping should probably be somewhere between old-school and the newer games. I like having control over my jumps but since SotN, jumping has been overly floaty for the most part.
I would say AOS/ OOE handled jumping very well, as well as backdashing to chain combos. SOTN's jumping felt overly floaty, but I also feel that reflected its slower pace of gameplay, which was slower than most classicvanias.
And if it's a home console game, then they might as well go all out; I think it would be awesome if it was a re-telling of Simon's entire story- yes, again. A 'definitive' version, if you will. So the game would be divided into three sections. First section is obviously the first Castlevania, but since the game is in three parts I'd tone the difficulty down a little bit.
The fun comes in where you get to completely re-work the Simon's Quest part of the game. I've always felt that Simon's Quest had a lot of potential but was poorly executed. Many of the metroidvanias have features that would have vastly improved Simon's Quest, like warping. That is the first thing I would add; you can warp to towns, and once you hit the end of a mansion, you have the option of warping out. By removing required back-tracking, this facilitates the implementation of more complex/difficult level design. The game would have more unique areas, every mansion would have a boss at the end that you could not just walk by, sub-weapons would be more diverse, NPCs could still lie to you but you'd be able to gain an optional item that would let you detect when they were lying early on, nothing absurdly cryptic to the point where it's impossible to find without a guide. No levels, no purchasable whip changes because that's stupid. Leveling encourages enemy re-colors and can imbalance gameplay; Simon shouldn't be getting stronger, he should appear to be growing weaker (because of the curse) by making the enemies progressively more dangerous. The game should be challenging; Simon is freaking cursed, and it should show..
I like this idea, though I had an idea in the past where there was levelling up, I feel that in some sense levelling up is more rewarding than learning new techniques. I'm not certain why, but I always feel more satisfied playing Castlevania's where one could level up. My initial idea was this:
Level 1-10 is the leather whip, 10-20 the thorn whip, 20-30 the morning star, 30-40 the flame whip (I know I've missed the chain whip, but rarely do players need to level up to L50)
My opinion would be that on the normal mode of the game you can level up, on hard mode, only your whip changes when having slayed enough enemies, on the hardest mode you can not level up the VK and you're stuck on Level 1.
i.e. same as POR and OOE on the hardest setting.
Like CV64/ LOD, including the difficulty settings to have harder/ faster and more difficult platforming, the harder the setting.
Include a day/night seamless transition, whether inside or outside mansions, which makes enemies actually change and take/ deal 1.5 x or 2 x damage.
Perhaps include a nightmare mode where it's like the hardest setting, but Simon just keeps getting weaker, the longer the game is played. Until the point where he will eventually die from 1 hit of any enemy.
No checkpoints mid way through bosses, for the love of god. Depending on the difficulty should determine where/when/how these are included. Nightmare mode should be more like oldschool gaming, no checkpoints, just continuing from a save point.
Adding this mode means casuals don't have to wet their whistles with frustration and discipline which the more hardcore gamers thrive upon.
Of course the harder the mode finished, the better the ending, the hardest mode should have a final form of Dracula which is unseen in other modes.
And my final change would be this; after gathering all the parts, whoever told Simon to collect the body parts steals them in a throw-back to how he steals Alucard's equipment, and revives the castle. Simon has to work his way back to where Castlevania is. Ta-da. Super Castlevania IV section, but it works more harmoniously with the story, and this is the point where you can really crank up the challenge because at this point Simon is still cursed but he's trying to do what he did in the first section.
I would say rather than stealing body parts, Dracula just tells Simon that he's too late, reassembling his body has re-awakened the Castle, thus allowing Dracula to reach full power. Castle rises up around Simon, Drac flies off to the throne room, Simon has one final quest to take on.
Really solid ideas though, I like them, Simon's adventure is overdue for the next gen imo.