The Oxeye-developed, Mojang-published Cobalt will follow a path to completion not unlike the one Minecraft took. Though it's being described as a platform game, it appears to offer players quite a bit of flexibility.
For approximately $13, interested parties can grab Windows-only alpha access, which serves as an introduction to the single-player/co-op campaign, level editor, and local multiplayer. This will help fund further development, and yes, early adopters get the best price on Cobalt, in addition to future updates.
\ We've blogged about this before: read (4) back stories