Great work.
Cannon needs a slower rotation as shooting changes direction so it could be more controllable and allow more skill.
In Unity you can set the render to scale of the camera which would allow the rotating barrel to fit with the background resolution. Also the cannon barrel could be placed behind the rabbit ball.
Maybe some kind of radar to give the players more of an idea of where the next threat/target is coming from.
Potential power ups control cannon direction, two cannons, three cannons, machine gun with limited ammo, faster shooting, bombs that arc and explode on impact.
Actually, could this become a 3D static turret shooter game, or Rabbit tank game.