I have a couple things that I think would be fun or useful to use in MineCraft.
Blocks: Wet Sand - Sand that doesn't fall when it's suspended in mid air. This would allow for easier construction of Sand Castles or pyramids. Moving Blocks - Self-explanatory. You could make elevators, conveyor belts, escalators, etc. The downside would be that you would have to set where the blocks would stop. Fire Blocks - Of course they would have to be animated to be better looking, I just thought that, along with the torch or lantern idea, you could use them for lighting, or burning leaves. (That could be bad, though, but who doesn't like a little destruction.) Bad thing for Multiplayer would be griefers going around burning everything. Just a suggestion. Mirror Blocks - Basically blocks that you can see your reflection in. This could be used to see your own skin, or whatever you wanted to do with them. Different Colors - A darker blue, Maroon, solid black, and other colors for the "cloth" blocks that they don't have right now. Rope - For bridges, obstacle courses, etc. Elastic - You could bounce on it, if you made, say, a trampoline. Hard Rubber - For lack of a better name, Hard Rubber. Whatever Diving Boards are made out of. You could bounce on them and they could making you jump up higher than you normally would be able to. Chain-Link Fence Blocks - Just for looks. Maybe you want to fence your house in? Make a warehouse-looking building? (If you don't know what a chain-link fence looks like, click here.) The door block suggestion could go with this. But as a gate.
Game Modes: Destruction Game Mode - A game mode for people who've bought MineCraft. (Like me.) It could be either Single Player or Multiplayer. In Single Player, which could also be put into Survival Mode, you could shoot rockets, use explosives, or simply use shovels and the sort to destroy buildings and other things that have been constructed. All of the explosions would have to be animated, as well as smoke, the missiles flying, and other things, though. In Multiplayer you could have servers just for this game mode, the servers would be reset afterwards, obviously, but there could be two teams and each team would have to make a fortress in a certain amount of time. Then each team would have one turn to fire a missle or whatever weapon they are using, at the other team's fortress to try to destroy it. (It would be turn-based.) Capture The Flag - There would be up to four teams at once, and they would play capture the flag. They could make their own bases or use pre-made bases. There could be weapons, vehicles, etc. For those of you who don't know what Capture The Flag is, the game is where there are two or more teams of a certain amount of people and they have to run out and try to capture the other team's flag, and bring it back to their base to get points. Battles - Couldn't really think of the name of this, but there would be battles between teams using weapons and vehicles. Whichever team that has the least amount of players at the end loses. The game could also end once one team doesn't have any players left. There could be bases and such.
Miscellaneous Ideas: Vehicles - Some times there are large distances you want to travel but don't feel like moving that far, because it usually takes quite some time, depending on the distance you want to travel. Maybe there could be vehicles or animals, such as a horse, in the game. Such as a car, a dune buggy, etc. Speed - In some games, the more you run around, the faster you can move. I thought that maybe this could be put into MineCraft, and it would make for more challenging games of tag or Spleef. You could also see another players statistics, such as their speed, skin, date joined, etc. Stunts/Tricks - Some people have made pools with diving boards. It gets boring just jumping in. Maybe there could be stunts or tricks that are set to a hotkey. There could be diving, a cannon-ball, etc. On land, you could dive and then roll, like if you are playing in a battle (see the above in "Game Modes",) to dodge attacks.
I might update this post if I think of any more ideas. Tell me what you think of them.