I don't go into fanboy rumination mode often, but I think I have an idea that rules (don't we all!). It's actually been kicking around in my head for several years. Mercury Steam- read this, and make this game. Thx!
I'd love to see a remake of CV1 & CV2, as one game in 3D. And simply call it Castlevania: Simon's Quest. The entirety of Castlevania 1 could sort of parallel the introduction of Symphony of the Night, where you are basically retelling the previous story as a playable prologue. The only difference being that it would start at the very beginning. Something you can jump right into and start whipping things before you're bludgeoned with the story. You could toggle tutorial-mode explanations on how to play the game too, something that'd be nice to get out of the way before you feel like you've started the "real" game (which would focus on CV2).
Compared to the grandness of worlds in some video games today, it'd be entirely realistic for a *relatively* literal adaptation of the NES CV1 Castle to simply be one large, linear level. It's something I can envision as being done awesomely. And just like other games with long playable 'intros' (Final Fantasy 1, Metal Gear Solid 3...), it could have that nice "wow I'm just getting started?!" moment. The Ending of Castlevania 1, with the Castle collapsing and all- could serve as the introduction to the story. That's where you could explain the passage in time, the curse inflicted upon Simon... and how his powers have made ordinary people fearful of him and turned him into an outcast, etc.
To bring back some RPG elements and a greater sense of exploration, I could see that being done in one of two ways for a CV2 remake. You could either have a large, free-roaming outdoor world while the mansions are more like traditional levels when you get to them. Or, vice versa-- the outdoor portions could be more linear like LoS, but have the mansions and towns be these huge, explorable things. I'm confident there would be room to combine both metroidvania and linear levels in something like a CV2 remake.
I'd also be a fan of introducing more weapons and lots of equip-able items. Anything to help bring back that sense of being overwhelmed by details and options that SotN gave us. I see no reason why a Belmont can't use a sword AND a whip. I know LoI tried to throw in some explanation for why Leon's sword was taken away, which only helps strengthen the idea that a Belmont should know how to use other weapons too. Castlevania has become a world with so many different weapons and means to kill enemies... as long as we keep the Vampire Killer as the "main" weapon of the Belmont's, I don't see why a Belmont has to be the most restricted type of character.
Unlike my idea for the introduction, I wouldn't want a CV2 remake nearly as literal, at all. You could have a large world to use in which you could recycle the best ideas from previous Castlevanias, as well as innovate. That era of CV is so loose in story that you could have it fit in the new timeline or the old, it doesn't really matter. Different Vampire lords and other baddies could rule the mansions and have their own little stories. The mansions could be drastically more themed, kind of like the Rock Castle, Plant Castle, etc, from Belmont's revenge. We could see the fear that people have in Simon is based on a "legend" that states that Dracula was... (spoiler- insert LoS reference here). There could be a secret area with Alucard's resting place (assuming the story between CV3 and SotN is still intact), where you jack his 'Alucard' weapons. There could even be a literal 'graveyard duck' this time around. lol

This is why CV2 gets my vote. It would have a lot of freedom to be amazing.