There are a multitude of theories saying that Minecraft is set in the future due to a plague mutating creatures, causing Villagers to have conjoined arms, and creating Creepers. However, in the words of Jamie Hyneman, "I reject your theory and substitute my own!" I have a multitude of theories myself. I may even be able to connect all of them. Here we go...
Theory #1- Steve is a Psychopathic Murderer Edit
Here is the gist of Minecraft- make a house, get food, mine, find diamonds, travel between the two dimensions, and kick ass while doing so. However, no one seems to consider that although the world technically belongs to the player, and the player technically has control over who lives or dies, some may abuse that power by just deciding to kill any and every mob unfortunate enough to stand between him and his diamonds. Steve's obsession with finding diamonds will only make him stronger. Strong enough, in fact, to make him an OP'd ruler of the entire world. Steve will use that power to make him the ultimate ruler over all creatures.
One must also consider why Steve is psycho. It could be the stress of being the (not really) only person in the game catching up to him. Steve could also not be accustomed to a societal structure. Since Steve can't conform to a society that doesn't exist, he doesn't know when to stop killing people.
Theory #2- Steve is Actually a God Edit
My previous theory talked about how he becomes OP after he finishes his quest for diamonds. That ties into this theory- that Steve is actually immortal. Here are some things I considered before making this theory- Steve himself, Steve's creator, his digitized life, and his rebirth.
First, let's talk about Steve as a person. He is around the size of a normal human, give or take a few pixels. Get it? However, what's interesting about him is that in an infinite world with no way of clothing (save armor) Steve has a blue T-shirt, jeans,and shoes. He has his own clothes in a world where there are no sources for them. A bit of a contradiction, really.
Second, one must think about Steve's creator- Markus "Notch" Persson. "Notch", according to UrbanDictionary, is another name for Jesus. Jesus- the divine creator.
Wait a second. Minecraft has a "creative" mode where Steve is able to do whatever the hell he pleases. One can also create buildings even in "survival" mode (if he so desires). Is this the "spirit of creation" that Markus transferred to his star player? Was Steve designed to be Jesus?
Another thing about this part of the theory is that Jesus saved the world, just like Steve is theoretically supposed to save the world from the Ender Dragon.
Back to the main point- Steve's life. When Steve crosses between the Nether and the End, he is passing through time and space. He is crossing in between borders between his world and others. Only a god would be able to do such a thing.