Tip: You can use Lunar Magic's Overworld Layer 3 Editing system to edit or create your own layer 3 images (castle bricks, fish, rocks) and apply unique effects per level via the Layer 3 Bypass in the main editor.
I suggest making the Koopa stay inside the shell or make it customizable. Also, I always loved
Shady Koopas from TTyD Tricky lil' bastowds
Perfect for a Pit of 100 trials-like dungeon/cave-like level.
Oops, gave away some ideas
I know that there's a tool (that I used to have) that makes Koopas stay inside their shells, so I guess you
don't need to edit the sprite Erik after all, if you don't want to. I still suggest it though.
I mean, it should be easy for anyone to look at the code for the Koopa and edit the ASM of
this sprite I guess... If you're okay with that, that is.