Just an idea on slimes... I think it would be a great idea that slimes awould be able to grow to the size level above them until the reach huge. But instead of just periodicly growing they could grow when they move over something e.g. the new rotten flesh, droped by zombies; they would pick it up in the same way as the player but it would instanly increase their size. In this way you could feasibly farm slimes for the production of sticky pistons.
Mummies in desert regions, they are similar to zombies, walking around with outstretched arms. As noises they just have a muffled mumbling like their mouths are held shut by the bandages and they try to yell something at you.
A little gag idea was that they can't see and therefor always walk in a straight line unless you make a sound (break blocks, jump, walk [they can't hear you while sneaking]) or they run into a wall.
When defeated they poof into dust, drops could be bones and a rare chance they drop lapis lazuli or gold.
-bigger more impressive mountains, and more likely to be snow-capped
-mountains ranges, maybe even a mountain biome
-volcanic biome,with a new block called pumice(type of volcanic rock)
-structures like stacks,and stumps and arches
-more types of rock e.g granite
-ancient temple ruins
-larger caverns with stalactites and stalagmites and underground rivers
and lastly, not really a landscape idea but what the hell... bats(in caves)