Notch's new game is a hard sci-fi something or other called 0x10c.
The truth is that I'm not sure exactly what Notch's new game is in any specific sense and I have no idea how to pronounce the title [although I suspect there's some kind of phonetic play going on here; Zero Extents, perhaps?], but it sure sounds like it'll be something... well, I don't know. All that Notch himself has said about 0x10c is that it's new, it's a space game, it's "quite ambitious" and there will likely be a monthly subscription fee for online play.
The game will take place in a parallel universe in which the space race never ended. Thanks to an unfortunate miscalculation that smacks of forgetting to convert to metric, what I assume was the entire population of Earth was put to sleep in 1988 and didn't wake up until the year 281,474,976,712,644 AD [not kidding here], with the entire universe teetering on the brink of extinction. What happens at that point isn't really clear but the dev team "hopes to include" lots of hard science and engineering, a fully-working in-game computer system, space battles, duct tape, seamless planetary landings, an advanced economic system, single and multiplayer action and a whole lot more.
Whatever it turns out to be, it sounds like 0x10c will be aimed first and foremost at the techno-wonks in the crowd. "The computer in the game is a fully functioning emulated 16 bit CPU that can be used to control your entire ship, or just to play games on while waiting for a large mining operation to finish," Notch explained. "Full specifications of the CPU will be released shortly, so the more programatically advanced of you can get a head start."
0x10c is still very early in the development process but the plan is to release it early, much as was done with Minecraft, and let the players help guide the development of the game. More information about the game, including a partial DCPU-16 spec doc - because nothing says "tons o' fun!" like "In this document, anything within [brackets] is shorthand for 'the value of the RAM at the location of the value inside the brackets'" - is up now at 0x10c.com.