Maybe some cosmetic/furniture/weapon drops that fit the the mini bosses for some mobs. Like a bear rug from the bear boss, grey version from cave bear. Tiger/Alpha wolf hoods and jackets. There's many possibilities that could be added. One thing I worked on a bit was a kunai fashioned from the fangs of the Rat boss in sewers but I kind of gave up because I'm not that great at pixel art.
Something else I worked on in the past was a Candy Cane back item for christmas but once again it's just a single frame of art, I'm not skilled enough to do every animation needed. Another idea I had was a back shuriken made of ice.