
A couch cushion. Now that would be great. 4x4 or 2x2, maybe? Would help people to make more detailed couches.

A colorable stone block would be great for castle building. 2x4 size and perfect.

A crate would be liked by many. There's probably one out there already, anyway, but still.

A vase would be great for a normal house. It would work very well for any fancy house.

A shopping cart would be fantastic. It would help people to make grocery stores, though it might not be used as much as the others above.
"
These, I believe, would make great blockland bricks." - CharlesSpeaking