Minecart chains might be a nice addition to Minecraft. They are crafted with three iron nuggets horizontal to each other.
When used on a minecart, a clink sound plays and the chain is attached to the minecart, much like a lead attached to a fence. Using the chain on a different minecart will play the same clink sound and connect the two minecarts, and using a chain on a minecart and using the chain on the same minecart will remove the chain. Clicking on a chain connecting two minecarts will drop the chain and disconnect the minecarts.
Oh, and if you use a chain on a minecart and then move, the chain will pull the minecart as well.