Idea: Tamed bulls could be castrated to make oxen. They could pull a new type of cart, the covered cart, that can hold more than the handcart.
It might hold 8 liftable items, and has inventory space similar to a cupboard as well.
Two oxen are required to pull it, with a beam and reins. The driver sits on the wagon, it has space for 1 passenger.
Up to 2 tamed animals can be chained behind it, by right-clicking on the cart with a chained animal in hand.
It would move at speed 2 on most terrains, speed 3 on roads. It cannot cross swamps or deserts. Ideal for those expeditions deep into uncharted territory.
Covered wagon: 12 blocks, 24 planks, 10 cloth
Beam: 1 block
Reins: 8 pieces of leather, 1 rope
EDIT: a side effect of making oxen by castration would be that you can castrate you lower quality oxen so they don't reproduce, and keep them together with you cows without any problems.