What is it? Let's face it; coal is the most abundant and useless of the ores. Although it is fantastic for working with meats, two ores, and torches, its abilities end there. However, simply eliminating this humble block would bring the game to its knees; mass-producing many items and lighting large areas would be next-to-impossible! However, there is a way to make coal one of the most valuable blocks in MineCraft. How?
Let's digress for a moment to the most prized item in the game: diamond. People have died over it, it shapes our lives, and everybody's MineCraftian lives revolve around this supreme gem. The rarest item in the game, diamond is, to be frank, pretty frikkin' awesome.
So now we have our coal, the useful yet crappy item, and diamond, useful and fantastic. Let's pull a switcheroo!

Wait, WHAT?!?!
No typo, switching coal and diamond's generation will definitely change the game. How?
1. Coal will become so rare that it is more valuable than DIAMOND.
2. Seriously, this will make the game much harder. This is only for pros. Lighting your home will become a challenge to be in awe of.
3. Conversations like this will happen often: "Hey Bill, can I borrow a diamond chestplate?" "Sure, Joe, take your pick, I got a whole chest of 'em in the basement."

How can I make an iron pick for diamonds without coal to smelt iron?
This is a problem easily circumvented. Use wooden items or charcoal!

Charcoal's easy to make, why not use that? Gotcha now, Kirby!
Thought you had me? Guess again, buddy! This mod would add a new item: ashes. When you burn items like wood or fences, instead of producing charcoal, they'll produce this new item. Craft nine of them for a piece of charcoal. Why not smelt things with wooden items? You can't make torches without some form of coal!