Ok so this is my idea. There would be a spaceship that you can craft like an iron golem, and when you get in it there would be several levers, one for each planet/moon and when you flipped the lever of your choice you would lose control of Steve and the rocket would take you to that planet. He would automatically walk to the window and just sort of look around but in reality that is just a loading screen like for the nether. The loading screen wouldn't end until the scene had completed and it looked like you had landed on the designated planet. Once there, it would be like coming out of a portal in the nether only the portal is the rocket. On each planet there would be aliens as mobs instead of monsters. Some may be hostile some won't be. They can give you something special when they die or they can just give you a lot of xp depending on how much you intend on working in that area. To leave you would flip a lever (Earth lever unless you wanted to go to another planet instantly from the planet you were already on.

Pros:This would improve the game by preventing too many drops from one mob if the you wanted to spread out the mob drops to increase difficulty on hardcore or any level. It would also improve the game by adding more adventure and creating a new big goal. There would be more themes for Youtube videos.

Cons: Tell me if you can think of any in the comments. You can't go wrong with Minecraft! But seriously tell me if you can think of any so I can fix them. Make it a challenge to find one I can't fix.

Additional Details: 1.Gas planets like Jupiter may or may not be possible to go to for obvious reasons.
2.The planets would be just like the other two worlds simply with a different design, nothing too detailed but not boring.
3.Aliens could be common mobs with different skins, but I believe most people would prefer additional limbs or something.
4.When losing control of Steve, you stay at the 1st person view.
5.Even if loading a planet has finished, you will not be able to play until the rocket has landed.
6.In the unlikely event that the video being played while loading a planet finishes before the loading does, you can slow the video down based on how long it takes to load the first 5% etc..
7.Making Mercury/Pluto a cubed planet instead of infinite would be nice but possibly difficult because you would have to change gravity when you cross a corner.
8.The rocket should be easier to build than beating the Ender dragon because there is a big gap from building your first nether to defeating the Ender dragon or making a beacon or making a wither.
9.If the rocket is going to be a goal you can finish before slaying the Ender dragon, then only the legs of the rocket ship would be iron and the rest wool.
10.The rocket in the picture is a rough sketch, the finished model could vary greatly.

P.S. This is not currently being worked on as a mod, it sounds like an unfinished mod so that you can imagine what you want to see and not what I want to see.