You can make far leg less contrast, that way walk animation will be easier to read. Currently your robot has very bright arms and legs, while its body is coal-black - is that your intention? I would have made exhaust steam a whole cloud, because it could look noisy in game. Also, palette could be tweaked too.

Is that one foot and one spike? It is incredibly hard to read the spike because the arm behind it has something interesting as well. Perhaps you can alter the design so that these two elements do not overlap.

And is that a backpack? I would think that a backpack would move as a whole, unless it was filled with something really light close to his body and something really heavy in the far end of it.

I think there are many large and small things to do with this, but if you're interested in working on this for a while lets see first where these few ideas get you!