Honestly, I can't think of anything that a sequel has that the first one could just add in updates.
Better graphics, maybe? Then again, I kind of enjoy the pixelated 3D world. It is like it has its own art form.
Added (1). To be more specific, this is a hypothetical question if there was going to be a sequel.
I don't think it will ever get a sequel, I think everything that is new will probably just be an update.
Machines! Getting bored of all the "new" mobs they're adding, most of the time they're useless. I want something more advanced, so machines would be perfect.
I doubt there would be a minecraft 2, but as far as minecraft updates are concerned i'd like to see more items and craftable objects and blocks. For example, rope, furniture, some kind of cannon or ballista that you can use to defend your house, more minerals like steel or copper. Mostly, though, i'd like the ability to go so high up that you can mine the moon. Get moon rocks, kill alien like creatures, etc. Also would like to see dreamland be made into reality. One of notch's old ideas, it was the heaven equivalent of the nether, a floating city that you can occasionally visit when you sleep in a bed.
I would a final boss to the game