A new dimension would be nice like the end or the nether except this one could be the moon you would have to travel around minecraft looking for something rare like moon rocks that spawn 1 to a biome but there is a 1 in 20 chance a biome will contain one and it would be the same structure as a nether portal except with moon rocks.

this is simple more ores. I would love one rarer then diamond some in the nether and some found underwater only. An ore I would love would probably be Jadeite selling at a real world price of 3 million a carat.

Minecraft's sea is huge but very blank.
Minecraft should add something that is very interesting. A reef perhaps they could add coral some visible fish or more fish to catch then the plain grey fish. They should add a Atlantis like underwater ruins but it cannot be like some five year old made it so that the chances of spawn are1 in 10 how about 1 in 100 and it should be deep enough there would need to be a scuba diving helmet and you would have to place it in your head slot. Then you would have to go down about 50 blocks to get their. There could be chests that can be made for underwater so you could find some artifacts and armor and tools you could only find there.