What your ideal concept for a Castlevania clone/sequel that's never really been tried before.
I have 3 in mind:
>1. a Classicmetroidvania like Simon's Quest but with all the bugs worked out and perfected
I don't think anyone anywhere has done that.
>2. a Vampire Killer Castlevania 3 hybrid
This is another one that I don't think anyone has even attempted. Being able to fully explore each level and also having alternate branching levels would be a really nice touch. Curse 2 kind of hinted at something like this in its design and frankly it would have turned out to be a better game if it had.
>3. a Japanese themed Castlevania clone
I would be fine with either a Classicvania or a SOTN clone. I imagine it would end up playing something like a Nioh 1 or Sekiro demake crossed with classic Ninja Gaiden and maybe with a little Sengoku thrown in. And I'd be damn happy with such a thing too.
The best shot for a good 3D Castlevania would be if Sony finally relents and puts Bloodborne on the PC and then someone mods it and Frankensteins together Cainhurst, Hemwick, The Astral Clock Tower, the Lecture building and maybe parts of Mensis and Old Yharnam and some of the chalice dungeons and then fiddles with the weapons and hunter tools.