So i'm sure someone else probably made one of these already, but this is my idea of a realistic season system in Minecraft.

Lately in my survival game i have realized that i have stacks upon stacks upon stacks upon stacks of food, literally so much food its not even funny, i could feed a whole server for life with it. and i realized, that it isnt just my survival world, its every world, server, and LAN i have ever played, so much food. then i realized, what prevented people in the Minecraft age (like 1100's AD/ high middle ages) from having THIS much food. then it occurred to me. winter.
the cold, the dark, the uncertainty, the danger in the night.
they all worked perfectly into Minecraft, and it also changed the way the world would work.

so this is an outline of the seasons i will go into depth about their different functions later in the post.

Spring: the first season, the default season the game begins in when you load a new world. and a crossroads of winter and Summer, in spring rains are sparse, and snow begins to melt as it does in a desert biome if placed by the player, ice thaws slowly, and lakes begin to take their natural form again, and the taiga biomes still have snow. animals can also breed and crops will grow now. the days have normal length.

Summer: the second season of the "year" and the warmest, snow in ALL biomes melts, in the taiga, it rains instead of snows and all storms are thunderstorms. the days in summer grow longer, from 10 minutes, the day now lasts 12, and the night only 8. summer is a happy time, flowers bloom, animals breed, crops grow, but prepare your food stocks for what is to come.

Fall: the third season of the "year" moderately cold, the snow creeps slowly back into the taigas, but it continues to rain in all other places, in fall the day slowly shortens back to the 10/10 cycle, all effects of fall are similar as those of spring, but fall is succeeded by winter. day is normal length, crops and animals are normal.

Winter: the anticipated season, always prepared for all year. if you are not prepared you WILL not survive it.
all biomes snow, all the time, no mater what. snow blankets the ground in wafts of white. all planted crops pop off the soil into their respectable items (wheat-seeds, melons-melon seeds, potato plants-potatoes) crops cannot be planted and will not grow. animals will not breed, nor will they follow food the player is holding. the days grow dim, the light at noon subsides to that of during a thunderstorm at all daylight hours. the night creeps longer, and slides by at a snails pace, from 10 minutes long, the night now lasts 13, and the day a mere 7 minutes of dimness.

winter is a dangerous time, monsters live around the clock, and your food supplies dwindle. but just as you seem to lose hope, the days become longer, the first rains kiss the ground, and your plants begin to grow as the sun breaks the freezing clouds.
springtime is back again. but your time to relax is cut short, get to work preparing for the next winter, make it better than the last. survive, thrive, and beat back the cold.
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The Average season will likely last between 15 and 20 minecraft days. summer and winter being longest at maybe 20 days each. with fall and spring lasting maybe 12 minecraft days. making the minecraft year about 64 days (hehe)

other ideas: a hard-mode version of the mod, which would include an unpredictable event known as "frost" a day of winter like conditions in spring and fall. meaning, all the crops you plant die due to the frost, and must be replanted.

Summer desert: in the summertime, if you do not have any piece of headgear on, you take -1 food to the already used amount, and -1 damage (half a heart) per second that you are in the desert in full sun (tick to see if sky is visible overhead would trigger this effect), with headgear on, the damage to health is gone but the food damage is increased to -2 added to normal usage (due to the fact that you get tired and overheated with clothing on in hot weather)

The Flooding Swamp: in swamp biomes, the terrain is extra gooey and muddy, while traveling on swamp peat (the ugly dark green crud in swamps) you will incur a -3 to food, and a 20% reduction in movement speed, so about half the speed of soul sand. because people don't just stride across bogs, they are dangerous, wet, and humid.

Frozen Tundra: the tundra is a harsh and inhospitable landscape. while living in the tundra, be it extreme hills tundra, taiga, or just plain old snowy fields, your player will take -2 damage per second while walking (because bare-footing it in snow is a bad idea), if leather boots are equipped (any metal or diamond counts as barefoot because metal isn't a good insulator), the player will only take -1 food from trudging in the snow all day, no damage will be done. HOWEVER. in wintertime, NOTHING will protect the player from the freeze of the tundra. they will take -2 damage (1 whole heart) no matter what while outside in the winter tundra. there are benefits of living in this climate though.... the permafrost layer in the soil protects your plans, and during winter time, your plants will not pop off the soil (so they will stay planted) but they sadly will still sadly not grow. the frozen tundra also has no frost days in spring or fall.

My idea will further the gaming experience and not really take away too much for the sandbox style game. this mod would be beneficial to the game play of minecraft for the true survivalists, who actually crave the idea of a harsh environment. to roam the sandy dunes in search of treasures buried beneath, to brave the muggy jungles for dangerous creatures and forbidden spices. to run across the tundra, and climb the highest peak. all the while dreading albeit with excitement, the next untamed landscape in which to explore, conquer, and thrive.