I'm pretty sure this isn't new, but how about making farming, fishing, foraging, animals also seasonal? Remembered Harvest Moon. So yeah, certain crops, fishes, forageables would only be available in certain seasons. Crops also probably affected by "ambient biome temperature"? So crops grown in x biome are better than crops in y biome? This obviously has an effect on market prices, so how about adding Food Spoilage and Food Preservation system to deal to balance out market seasonal crop hoarding if the above is implemented?
Since there's nothing to do in Winter right now, how about wildlife that only spawn in Winter? Something like winter wolves or polar bears that try to break into your base by making a hole in your wall(not literally destroying a whole wall affecting PVP) and kill your animals? Penguins? Reindeers becoming more deadlier in the Winter? How about a VERY rare encounter like a Yeti? An Ice Dungeon?
How about a hazard like a snowstorm which affects certain areas on the map? Needing to keep warm during the Winter, which adds additional purpose to light sources?