This has the start of something that's Tetris-like in terms of strategy, but I found that mashing buttons and dumping as many crates as possible onto the mule was about as effective as carefully planning and deliberately trying to stack crates.
It would be really helpful if you could give us the next five block types so we can more carefully strategize which ones to place. Also, if you give us more options like the ability to discard a few blocks, it would lead to a greater depth of interesting choices the player could make.
Some humor would really take this game over the top. Maybe you're stacking crates full of animals or something funny / silly that's hilarious when it collapses