Heads-up - this is obviously incomplete. I'm sharing it now for other people to build on. This engine is open-source, and as you can see, there's nothing to get in the way of your own art.
It's currently a 16 KB NROM. CHR is mostly junk. One nametable is untouched. There's plenty of room for whatever you want to make. (And plenty of room for improvement. I'd recommend picking a mapper that can move more tiles onscreen.)