At this point, the phrase "open world sandbox" practically has no meaning. Toss in a fully destructible environment, tons of building materials, and some tidy graphics that are more than just pixel art blocks, though, and you've got enough to turn our heads. Throw in co-op and we're pretty much sold. Yeah, I know, we're easy like that.
Rising World recently hit Steam's Early Access, and so far the reception has been good. The game follows the usual Minecraft pattern of dropping you in a randomly generated world full of breakable blocks, handing you a few simple tools and turning you loose to see what you can build. The difference here, though, is that the world is a touch more realistic. Not only does it look prettier, but everything you see can be modified, whether in-game for via server-side scripting. No restrictions, just, um, a word that means "freedom" that rhymes with "restrictions".
Quick list of some of Rising World's features:
Randomly generated worlds Completely modifiable environment Both single and multi-player modes 200 different building materials Place customized images in your world Complex and deep network of underground caves Environmental physics - trees animals and multiple other items Interact with just about everything, right down to playing the piano! Day-night-cycle Server-side script support (LUA)Co-op support lets you run your own server so others can join in and play. It requires a little setup to work properly, but hopefully that will change as development continues.