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.
(Upon looking at the thread review, it's trying to load something, but it's failing, might want to fix that.)
Otherwise, this is a neat little thing. I don't think it's smart to work on stuff for polishing when space is already so tight of an issue. More Overworld Sprites can probably wait until after all the required stuff is in the game and fully functioning correctly.
After discussing with Lui and some others over IRC, these are the ideas we came up with:
-Slow Moving Cart on Circus map (like the cart in coaster commotion)
-Porcu-Puffer swimming around island on Japan map
-Randomly jumping up Podoboo on Fire/Ice map
-Windowing around mario that does STUFF in factory map (maybe it flickers, window expands as Mario beats levels?)