First of all, if they were going to make a Castlevania game they would have to make a real effort to seperate it from what they've done before. I do not want a Castlevania game that is really a Dark Souls 4/Bloodborne 2. I want something that learns from those games but is still very much it's own thing.
I agree with Shelverton that a reboot of Cv1 would be best. A completely minimalist story where you are Simon Belmont, marching on Dracula's castle to take him out. That should be the entire story, with no or very few cut scenes or story moments. I would love for them to take it back to the Barbaric Belmont style of the NES games, with the rest of the games aesthetic adjusted to fit accordingly.
Now, where people may disagree with me... I think they should go for a more sun drenched, golden mountains, white castle type feel. Think the covers of Cv3, BR, SCV4, etc... that kind of high heroic, almost swords and sandals look. Make the masonry almost greco-roman, retool classic enemies and bosses to feel more epic and fearsome, less cartoony (note NOT gruesome or twisted or any LoS nonsense). Of course night moments should be dark, but when the sun is out it should look like a Conan movie. This would do worlds for differentiating it from the previous games by From Software, and is one of many types of aesthetics I feel could do Castlevania justice.
Then in the next game have a different company approach the franchise from a completely different angle.