Here's what I think could have been improved:
So...you've made a Storm the House clone. It's very pretty, and it's very functional. Good job on that. However, there are a number of design flaws that keep me from adding this game to my favorites:
First, you made it so that the only dominant strategy in the game is to level up your pistol until you can get the sniper rifle, followed by the shotgun. This renders all the other weapons pretty much useless, including the support penguins. This is a common flaw in Storm the House clones, but a glaring one nonetheless.
Second, the difficulty curve is non-existent, unless you decide to switch to a weapon without upgrades. I should not have to purposefully make the game harder in order to have a good time.
Third, the upgrades are unbalanced. It becomes very obvious early on that reload times are the most important attribute in a weapon, followed by power, then clip size. Why, then, do they all cost the same amount? If you had made reloads cost more than clips, then I would have had more fun scrambling for the money to buy those precious reload upgrades.