Mostly merging and refining the DXC and Saturn versions. I'd want Richter in his goddamn SOTN outfit, a story for Richter (make it a prequel that explains how Shaft got the jump on him) and another for Maria (probably one concurrent with Alucard's take on events), and the additional areas, items, and enemies from the Saturn port (just not sucky this time, and much more sprawling in the case of the new areas). Also standard stuff like a resolution bump, dynamic lighting, and a remastered soundtrack would all work.
Of these, the only thing I'd
demand is an SotN outfit for Richter. A
proper one, not the shoddy one from the Saturn version. A Richter story would be really cool, and hell, they could even make his story just a retelling of Rondo in SotN's format (though everything from Stage 1 would have to be retold in the opening text crawl due to the lack of a town).
Actually, I'd like that. Richter's story could be a retelling of Rondo of Blood, ending with his "Die Monster!" fight from the beginning of SotN (though no cheap ass "RECKON YOUR STRENGTH" Maria saving you, and obviously a restructured final boss to make him actually difficult), and Maria's could be a story taking place between Rondo and Symphony that details Richter's disappearance, detailing what she did during Alucard's story, and her and Richter escaping the castle after Alucard ascends to the Inverted Castle.
Thus, you'd have a version of SotN that tells the entire Dracula X storyline in one game.
As for the new areas, yeah, I'd love if they were expanded and
actually served a purpose. As far as I remember of the Saturn version, you don't really get anything for going into them, and they don't even add toward map completion. In particular, a revisit to our good friend The Forgotten One could be added to the Cursed Prison.