Yoshi would be practically impossible to implement for two reasons. One, all the playfield characters are only 2x2 tiles big, or 16px by 16px (whereas Yoshi is twice that size in his widely animated incarnations, meaning ugly sprites probably).
Two, the palettes. Devil World is really strict with them without being able to hack sprites to use different palettes, which is currently beyond my understanding. For example, Mario, Toad and the Ice Flower all share one palette. Goomba Player 2, Luigi, the bonus round eggs and the ! boxes share another palette.
Mario World is pretty much complete, but I do appreciate suggestions and critiques. Maybe if all goes well and if there's any demand, I'll share some copies here on NA.