In the new update 1.7 there will definitely be new bioms, mobs, and structures.

Biomes:

Disco mountains: which bring canyons, overhangs and possibly new types of blocks

Large forest biom: Basically it's going to be like the red wood forests. Imagine jungle size trees, but forest trees. Epic yeah?

There are going to be sea cliffs. This is believe to be like beaches, basically giant walls of rock being vary dangerous to the clumsy players. (like me)

A new tree! There is more than likely possibility that a cherry tree will be implemented, which means there wee be new food sources! (cherry, cherry pie, ect)

Savan: A biom where its like grass lands but flat and has trees. It is thought that maybe new animals will be in this biom, tigers, lions, elephants, ect.

Other possible bioms would be: volcanoes, sky worlds, and possibly bioms solely devoted to dungeons.

Mobs: The most anticipate mob will be the over world dragon, notch tweeted that he would be working in the “red dragon”, so this is completely possible in the 1.7 update. Gosh I would like a pet dragon, I think I would name him alfred.

Of course their should be new savana mobs, but nothing has been confirmed.

There should be more more underwater mobs, because one of the developers(i forget who) said an reddit, that he need so work on how the engine runs because there will be more mobs which will create lagg. He was talking about underwater mobs.(That isn't exactly what he said),

Theres going to be a new fishing mechanic. Is this going to make it interesting?

Nathan Adams tweeted: “Happy with the new fishing mechanic. It's not something you would be doing every day, but it isn't something you would avoid”. I believe it's going to be a little more exciting, maybe adding bait?

So please comment your thoughts and what you would like to see in the new update.