The big one I'm waiting for is Starbound. Similar in premise to Terraria but on a much bigger scale, with a procedurally-generated galaxy to explore.

Also I hear good things about The Witcher 3. And technically I'm already playing it, but The Dead Linger! It's currently in early Alpha and I have access through its Kickstarter campaign. Basically a procedurally-generated zombie-apocalypse survival-horror/FPS. It's a bit ugly looking right now but even in the short time since the Kickstarter its gotten much better and the devs say that the final graphics when they reach v1.0 will be at least on par with the Stalker games (they quoted the 2nd game, Clear Sky in their blog). Definitely one to watch for fans of zombie/survival-horror/sandbox gameplay.