Carrying boxes does get repetitive. Something to solve that problem in higher waves would be cool (for example: "air drop" special ability: when you call it and mark an area, 5 boxes will fall from the sky at that position.
Sorry. I know I have way more ideas than you can implement, so you'll have to filter out which ones you wanna do...
~"I never let school interfere with my education"~ -Mark Twain