Thirst should of been added before hunger because we need water more then food and we could obtain this by going to a lake or ocean. This also includes drinking potions, water bottles or any liquid that is available. We can drink the water from oceans and lakes by right clicking then our thirst bar would be replenished.

More Mobs would be great so that Minecraft could be better and more exciting and attract some more attention.
(Not that it has't already) Which would make the game in some area's harder and in some area's easier.

Something to fly in would be bloody awesome so we don't have to build rails or go across water but you could make it harder to craft so it is not so easy to play the game. The ingredients for Leonardo's Flying Machine I think it would be is paper, string, sticks, diamond and iron ingots. The controls for this might be a mouse movement for the pitch and yaw and WADS for manual controls.

Lastly it would be pretty cool if there was body reactions to the type of terrain you are in. For instance if your in the desert if you stay there for a while you begin to sweat and have hallucinations. The way to prevent this is to have water with you in a bottle and or food. Also another cool environmental add on could be when it is raining water drips down your screen and keeps replenishing your thirst and flooding up areas.

Furthermore the ocean has to have fish or else it doesn't make fishing worth it or not so fun. Also it would be cool to see little fishes swim away while you swim past them going to a underwater cave. Also this would make the game seem more lifelike while fishing and swimming instead of invisible fish.

I believe Minecraft should have more civilizations as well as the villages. This would affect the game a lot eg. materials such as silk was big in China so i believe their should be trading or money could be added as well.

Please take notice of this and if you do thanks and I hope I gave some good ideas.