'Minecraft is a fantasy/medieval/whatever game so if that was added it would ruin the game.'
Which I agree with. Adding those things to vanilla would be stupid. But what if there were an entirely separate sci-fi version of minecraft?
It makes sense. Loads of developers have made futuristic 'alter-egos' to their fantasy franchises and wound up creating something equally popular. Starcraft is as popular as Warcraft. Fallout 3 was pretty much a sci-fi version of Oblivion, and is just as praised. Warhammer 40,000 now overshadows Warhammer, even though it was originally just a spin-off.
There are a few ways it could work. The one that seems to be most popular and that I like best is 4 so I'll work on that. The others are in the spoiler.
Space Steve lives in a time when the human race has colonised other planets. Instead of spawning on an overworld, you spawn on a large space station or spaceship. From there you can access several 'planets' with each planet being very different.
The gameplay would involve trying to establish a foothold on each world, with each new world you get to access being more hostile than the last. During alpha MC future would probably just have one world accessible, aiming to have two or three by the time the full version comes out.
- Coming soon!
To clarify a few things:
-This would be a spin-off so it would NOT change vanilla MC at all.
-In terms of development, Mojang has almost certainly got enough resources to put a person aside for a while to work on this. After all, the coding could pretty much just rework things already existing in MC. The only major change needed initially would be to combat and perhaps better AIs, so that enemy soldiers do things like take cover.
-I don't want to just make a custom map/use a texture pack because a lot of the fun of minecraft comes from exploring and the like. If I made a custom map, I would already know exactly where everything is.