I know there have been a lot of these popping up recently, but I had another idea: An entire re-imagining of Casltevania 2, where only the core of the game remains. You play Simon Belmont in a metroidvania action adventure and gather the remains of Dracula in order to resurrect him and break the curse he has placed upon you and your people.
Improvements:
-Well crafted, interesting, and smooth graphics. Increased resolution and colours.
-Multi-chip soundtrack
-Full save system
-Map System
-Equipment System, similar to that of later Castlevania titles.
-Movement system akin to that of Richter's in SotN, but toned down and more suited for horizontal movement rather than vertical.
-Entirely reworked map to eliminate backtracking and monotony.
-Clever enemy placement and better designed dungeons.
-Actual real bosses that you have to defeat
-No more cryptic bullshit, and no more relying on townsfolk for all your information.
-Night comes gradually, and has no message at all.
-More true difficulty, less fake difficulty.
Still deciding whether or not it's an idea worth going with.