After completing the Special World in Super Mario World, you unlock an Autumn theme, which changes sprites for the map and enemies. Unfortunately, Mario Maker 2 doesn't have this (unless its added in an update or it just hasn't been found yet) so a mod that adds these changes would be nice. And if you (the potential creator of the mod who might be reading this) wanted to, you could also add these changes to the other themes as well (it would be pretty hard to make but cool nonetheless)
In order to win the bounty, you should fulfil the following requirements.
Add the alternative Autumn sprites in the Super Mario World style