Some of my ideas! I will regularly add to this..
-More cat and dog colours
-More mythical animals, such as Pegasi, Unicorns, Griffins, Jackalopes, Small Dragons and maybe Winged Deer
-More animals like Deer, Moose, Foxes, Mice, Rats, Fireflies and other insects, Bears, Moles, Raccoons, Skunks, Possums, Worms, Giant Fruit Bats, Goats, things like that
<-Maybe a myth-inspired biome? Where you can find Pegasi, Unicorns and other mythical creatures
-Maybe an African plains biome, where you could find Lions, Hyenas, Zebra, Buffalo, Wildebeast, Leopards, Vultures ect.
-Fish that you can see swimming in water. Things like Piranhas (Hostile, will attack any living being or meat in water), Pike, Sharks (Hostile), Pufferfish, ect.
-Things like Seals, Orcas, Dolphins, Whales, Crocodiles, Narwhals, Beluga Whales, Walrus, ect.
-An Arctic ocean biome, with ice bergs. Orcas could jump, when you are standing on ice bergs, and kill you
-Chupacabras. Grey, wolf-sized, kills sheep and goats and has spines on its back. It basically looks like a wolf, just dark grey with blue eyes
-Sloths. Spawns in the Jungle Biome. Usually crawls on trees. Very slow.
-Little things, like Frogs, Squirrels, Chipmunks, stuff like that! :D
-Tigers and jaguars! Spawn in jungles, neutral
-Pegasi could be tamed with enchanted bridles? Unicorns, too!
-Enderrat. Looks like a black rat with purple eyes. Hostile. Spawns in dark areas.
-Giant Nether Moth. Hostile mob, common in the Nether.
-Tundra People. Like villagers, but they don't have big noses and crossed arms. They wear fur coats, herd Musk Oxen and have huskies. They live in quite advanced villages. They can have black, brown or blonde hair. Boys have short hair and girls have longer hair.
-Musk Oxen and Arctic Wolves. Musk Oxen are like large, hairy cows with big horns. Arctic Wolves are white, and when tamed, turn into huskies.
-Snow Leopards. Spawn in the Tundra and are neautral.
-You could tame the Giant Fruit Bats I mentioned earlier with any kind of fruit. Then you can mount them when you've tamed them.
-More fruit. Pineapples, Oranges, Strawberries, Bananas, Blackberries, Grapes, Raspberries, stuff like that.
-Underneath the Tundra Biome, you could mine Crystal Ore. You could make armor, tools and weapons out of them. With Crystal, you could make Crystaline Arrows. Tundra People could trade you for a fee of Crystals.
-Birds like Eagles, Hawks, Owls, Ravens, Pigeons, Sparrows and Storks. Eagles could kill rabbits. Hawks would be smaller than Eagles, and they could take Sparrows and Mice. Owls would hunt Mice and Rats. Storks would hang around water and eat fish.
-Gremlins and Goblins. Both one block tall. Gremlins could carry stone pick-axes, and Goblins could carry stone swords. They spawn in mixed groups. Their appearances would differ slightly.
-Fairies. They leave behind a trail of Fairy Dust. They could live in Enchanted Forests, maybe.
Sorry for any typos!