The palette order vaguely conveys a day-night cycle, but is admittedly rough. It needs fixing. As with the diagonals and cliff edges - it's programmer art. A flat-color version would presumably look even moreso. Somewhere between Hovertank and Behind The Iron Gate.
This engine specifically might get reworked for a first-person Metroid affair. Fewer enemies (with respawning), limited draw distance, some kind of level streaming. I do want to play with controls that rely on timing. Like a double-jump that's also a Half-Life long-jump depending on when you press the button, and a unified gun for both pew-pew-pew and BLAM. And definitely SNES controller support, there, because I'm not doing multiplayer again unless it's really funny. Like if Slaughter 2 has four-player deathmatch. I can't beat Faceball on Game Boy supporting sixteen people.