Already said it in a different thread, but what I'd love to see when it comes to blocks is having just one single type block you can place, stretch, rotate, and resize, to fit your needs, all by picking a corner of the block and dragging it somewhere else.
Grab a corner of the block and throw it upwards for a slope, or place several blocks next to each other, stretching them to make a sorta poligonal semi-circle for you to fight in.
The system can be as simple or as complex as you want it to be, depending on the stage you're trying to build. Want a simple straight line? Just place a block and stretch it to the sides, or even several square blocks in a row. Want a complex shape? Then you have almost as much freedom as you have when extruding boxes in a 3D modeling program, save for whatever limit in the amount of blocks they'd have.