Man, I'm a sucker for this art style. Great job on that front.
The idea is certainly interesting and creative, I wouldn't even have dreamed of this.
Gameplay wise — you already know it — there's room for improvement. But the fantasy is definitely there! Imagine filling up the walker with more and more machinery, making it tear through bigger and bigger waves.
As it is right now, I'm also primarily craving a) a more fluent system for actually using the different guns of the walker and b) being informed of the end of the waves / my performance and having the ability to go again, preferably with an ever-increasing amount of enemies. I'd really dig a game loop where you fight some waves, and then get some downtime to upgrade the walker.
But all in all — I really like this one. Great job guys.