Like the music, the physics, the transitions and idea behind the game. Sort of reminds me of angry birds but with zombies!
Cool features to consider: Have two or more shots per round where the physics on projectile ricochet could produce unimaginable and bizarre results. More dialogue story - it doesn't have to be complicated, think plants vs zombies.
Cosmetics to consider: having UI buttons to access the menu would make the game feel more at home; a restart button for quick access would definitely be nice - without having to press escape and accessing the game menu.
Other than that, unless you want to add explosions and such like in rocket league (for when the projectile hits a target) the game was pretty cool.