This upgrade would be bought and gotten out with 4. When holding boomerang press left/right click to throw. The boomerang will cause about the same damage as the bow, but after a certain amount of time, will come back to you and you will catch it. You can only throw the boomerang if you are holding it(like you would think).

Upgrades would include;

Bounce: this makes the boomerang bounce from one enamy to an other if it hits one when first thown

Boomerang size: makes the boomerang larger

Speed: makes boomerang fly faster

Fire Boomerang: The boomerang is on fire

Ice Boomerang: The boomerang is frosted(see more on my ice suggestion)