Spring and Summer are easily represented by normal biomes.
What I would like to see is a "Dead Biome" added. It wouldn't have to expressly be any specific season, there. Everything would just be "dying" and therefore would LOOK like Autumn.
- Red, Orange, Yellow, Purple leaf types on trees.
- Grass would be yellowish brown.
- Water would be a pale, eerie blue. (Think almost white...)
- Pumpkins and brown mushrooms would be widespread there.
In fact, let's go a little bit further...
Dead Biome mobs...
- Skeletons with pumpkins on their heads. Nothing new really needs to be added to the game, here. Have them shoot arrows that cause blindness?
- Zombies that are covered in mottled brown and orange patches of fungus and rot, called "Hallowed". Similar to the Drowned and other types of zombie. Basically just a re-skinning. Their touch could cause food poisoning for a more "You're turning into a zombie!" spooky feel without anything more than the existing food poisoning effect causing you immediate hunger.
- Hags! The current Witch mob with a reskinning to be gray, old, tattered, and so on. If you look directly at them, they're invisible. You can only see them out of the "corner of your eye", meaning the top, bottom, and sides of your game view. If you look straight at them, they fade from vision. Extra spoop!
Extra, unimportant bits of fun... (Don't consider these 'real' suggestions, but rather ideas of what COULD be done.)
- Every so often, the player might find lit jack o'lanterns on a single fence post. Who put it there? Creepy!
- Witch huts. Auto-generated, small wooden shacks with a cauldron and brewing stand. (Like the igloos, in a way.)
- If you want to just be outright weird, lava freezes in the Dead Biome. O_O Create a meta, creepy sense of dread for new explorers... "Is the game messing with me?!"
- Even more scary... if a switch is attached to a redstone lamp in the dead biome... it'll randomly turn itself off every so often. Very rare, so as to keep it startling when it happens and prevent unnecessary lag, etc. They only turn off, not on, so it doesn't create a never-ending loop of the game turning them off and on forever.
Basically, if we had a "Dead Biome" that could pass for autumn in its appearances, we'd cover all four seasons in terms of game aesthetics. Every day can be Halloween if you choose to build in this biome.
Adding a new biomes can be tricky with current world gen. This usually isn't a problem with small biomes changes such as bamboo forests being added to jungles. If this was added I suggest it to be added as a sub biomes of forests, birch forests, and swamps.
I completely agree (had a similar idea in the past). We need an Autumnal Forest, Winter/Spring/Summer are already represented in game. This is worthy of changing terrain generation. It could just replace 25% of regular forests on old worlds, if that's possible. If not it is still better than no autumn.
Not so much Nether as an area that's withered/blighted. It's just a way to explain why it looks like that, rather than saying "It's somehow autumn there". XD It would fit fine, imho. It's just an area that's not looking very vibrant.
The Meaning of Life, the Universe, and Everything.
An Autumn biome seems like a good idea, all you would need to do is change the textures of the grass and leaves, like in any other biome.
I like the idea of witch huts generating there, i like the creepy jack-o-lantern on a fence post, i like the pumpkins and mushrooms spawning there.
The ‘hallowed’ is a retextured, renamed husk. Nothing wrong with that per say but it’s already been done. The pumpkin head skeletons are okay but would be annoying. However, what about a headless horseman as a neutral mob? A pumpkin-headed skeleton on a horse that roamed about at night. It would use a sword and deal damage similar to a Wither Skeleton if provoked. It would only spawn rarely, attacking it or other skeletons would provoke it.
Maybe there could be an extremely rare chance to see a certain Minecraft Myth spawn? If under a full moon in a forrest, perhaps you will see a pair of pure white eyes staring at you in the distance. Herobrine? Or just your mind playing tricks on you?...