In reading the suggestion forums, everyone has an idea of what MC should be, for me MC has the potential to be the first open-ended RPG type of game ever created. And when I say RPG, throw out all the stats, all the levels, all the experience points, money, skills etc... that RPG's have become known for, I'm talking about completely breaking the mold and redefining what an RPG is.
Over the years what an RPG is has been defined by games like Oblivion, WoW, Dragon Age, etc... All of which are heavily based on a series of numbers to drive them along, especially WoW. It's the skinner box of get item A so that you can become more powerful and go acquire item B, so that you can get item C and so on. All of which is a system to drive the game along and in the case of WoW keep you paying to play it. And while games like Oblivion, DA, and Mass Effect all have stories to go along with their gameplay, the story is a slave to the gameplay, meaning that no matter how many choices that they "give you" you're still on rails to a predetermined end point. An open ended RPG doesn't have an end point or rails, just a framework for how a story can occur.
"Isn't a sandbox an open ended RPG?" In a word no, it's not. A sandbox is a purely open setting where you can do whatever you want, but the problem with that is that there is nothing driving you to keep going forward other than your own personal goals. Games do need some sort of objective, but that doesn't mean that they have to have an ultimate goal. Take Oblivion, sure it was a sandbox and you could do whatever you wanted aside from the main quest, but the main quest was always there waiting for you, and worse it's not really a sandbox if you can't mold the sand to your liking. Nothing that you did in Oblivion actually had any impact on the world that you were playing in. Take the side quests in GTA, do they actually have any affect other than giving you more money, or just stuff to do when you don't want to do the main story? Another problem with sandboxes is that all of the quests are scripted, just like the main story. Not only will they have no lasting effect on your world, but they are pointless in comparison to the main story that you're trying to "finish".
The difference with Minecraft is that it's the first game where you actually have control over your own world. If you need to make a house, you make a house, if you want to cut or plant a tree you do it, make a giant golden pe... you get my point, you can do anything you want in it. A lot of people like playing the creation part of the game, personally I like the adventure part of it, and building with purpose.
Getting back to my point, MC as an RPG can work if one thing was added, passive objectives. Meaning objectives that the game gives to you either directly or indirectly that can be done or ignored by the player, but without an overarching storyline. Below is an example of how I see that this could work.
On the forums I suggested master dungeons that control the spawning of mobs within a region of the world. These would work aside from normal dungeons in that the heart of this dungeon allows for monsters to spawn like normal within say 200 blocks of their existence, if turned off then no mobs will spawn in the area. These could be overlapping making clearing out an area a difficult task to accomplish, but it also give adventure types a goal that they can achieve. And those that aren't interested it doesn't affect them in the slightest, the game goes on like normal.
Let's say that you've cleared out a large region of land from all mob spawns, now what? Now from the chests located by those master dungeons or the hearts of them themselves you acquire a town beckon that you can place in the world which allows for NPC's to spawn and build buildings around it. So once an area is safe, people will settle in the area and create a town, and since you place the beckon, it shouldn't interfere with your buildings. With NPC's you can achieve the next step in an RPG, story telling, by creating a database of quests that the NPC's can pull from they can give you quests, but the quests themselves don't need to be scripted, they should be generated based on factors around the world. Examples: Find another town beckon so that the town can grow. Establish another town. Destroy another town. Heck it doesn't even have to require fighting it could simply be go discover a new region, or go trade A for B with these people in a distant land (maybe generate a random town or two once the first one is created).
In the end what you have is a game that progresses as you progress, there is reason to search for dungeons and explore beyond your spawn location. The towns once established give gentle suggestions on stuff that you can do around the town and depending on how complex the quests get it could develop into a series of towns that are at war with each other and you being the main catalyst, with no predetermined winner.
And the best part of all of this, if you don't want to do it, you don't have to. This whole progression works based on your desire to make it happen or not happen. So if you want to play MC as it is today then you can. But I could envision people writing quests complex enough that as they are randomly generated they could propel the player through a procedurally generated story unique to them, just as their world is unique to everyone elses.