My first vote is definitely for transportation of the animal kind. Minecarts and such are great but i feel like a animal would be much better. I am a huge horse fan and none of the games I have seen have cool horses. Minecraft horses are just so lame. You get on and off a couple of times and viola their tamed. Its just not fun. But horses with a actual taming process would be absolutely amazing.
I would absolutely love horses in the game. If you were to add them the should have herd mechanics. Like a stallion and a couple mares in a herd. The stallion would protect the mares from predators and lead them to the best grazing spots. To get to the mares you would have to trap/distract the stallion. You could also tame or break the stallion but it would be a lot harder. In order to break or train a horse you would have to start with getting them used to you, such as feeding high value treats, and just being around them. Once they get used to you, you can try lunging them in circles in a round pen. Then you could put a saddle pad and bridle on after a bit of lunging. You could then lung them with the tack on and eventually add the saddle. After they get used to that you could try riding them in the pen. If you have done all the previous steps right then it shouldn't buck you off. It could still just be a cranky horse and buck you off anyway. After a while of riding without being bucked of you could ride them around your world. I would also love to be able to breed the horses to get better stats like speed, stamina, and jump height.