I thought you are using A8. Sorry, my fault!

What I would prefer would be an update system instead of the current weapon system. You earn money for killed monsters.
I would make it like this:
The chainsaw is the default weapon for everybody (if you don't have ammo you can use the chainsaw). As addition, you can buy one of 3 start weapons. By purchasing updates for each weapon you get more damage, higher firerates, faster reload,... After some waves, new weapons are unlocked and can be bought.
I would also add ammo. --> is dropped by the monsters and needs to be collected.
But you should think about a feature to share ammo with other players.

It would also be great if you can carry more boxes if you upgrade your "strength" (but not unlimited, set it to max. 3 boxes or so). But you can only carry one metal box.

Just some ideas that came into my mind while playing.

I think such an updating feature is something a lot of people like. Just look at the thousands of tower defense and other browser games where all you have to do is upgrading ^^

I think this would double the fun of the game