My idea has been to come up with a plow that could be pulled by either a cow or a horse if they are ever implimented into the game.
This particular design could be used to plow out a larger area for farming, or very simply to flatten some parts of the ground within a 2-3 block tall range. It would not be able to go through mountains or climb above what an animal would normally be able to do, and you should be able to guide the animal where you like.
To make the plow, the pattern would probably be something similar to this:
or for a sturdier version
You could also possibly make it so that it can cut into the ground and move along, thus you could create grooves to make waterways or something.
It seems like a good idea when you first look at it. Maybe right click on an animal to place it, then use the directional buttons. Let me brainstorm for a minute though..
What was the reason the first plows were invented? But of course, they were invented to work the fields faster. Which means, if this is to be useful, then it has to cut through fields quicker than doing one block at a time with a hoe or shovel, which I could easily envision it doing if it was hooked up to a cow, horse, or mule.
You get my stamp of approval. Ten Notch's from Gryffindor!
I think plowing would be a good idea, but for it to work there needs to be an expansion on farming. There are few reasons for farming to be done and no reason for it to be done long-term.
In it's current state MineCraft has no need for a plow, but if farming was given more attention with a wide range of crops and tools and more goals to be achieved through it then it would be a good addition.