That would be a really fun and cool idea, but it would remove balance from the game. It would be a bit OP to have something like that, which would essentially render minecarts and donkeys useless.
Then again, Dinnerbone seems to care about this less and less. As much as I love the things he's been adding, along with Jeb, he really comes off as a guy who would rather add "cool" things to the game to appease the younger audience.
Then again, Minecraft, being a sandbox game, should have tons of stuff to do crap with and play around with, but it still annoys me when they add something that cannibalizes an existing feature, like horses and pigs for instance.
So as much as carriages would be cool and fun, it might not be a practical addition.
