When Christmas comes along, I would like it if Arctic biomes changed into Christmas biomes! A biome that contains many Santa's Workshops (A special dungeon, without enemies and more rewards, but you have to find the hidden loot), Christmas trees (chance of presents under them), Chirstmas-related quests, and flying reindeer!
When it's Halloween, it would be pretty cool if Hell biomes turned into Halloween biomes! A spooky biome that includes tons of Halloween Pumpkins skattered around, scary mansions as dungeons, and ghouls and monsters lurking around.
BTW, all biome changes would have to be temporary