The giant whale with arms(!?) and cyber legs shooting people and buildings is such a cool and wild design! It really sold me on this game. I think you already know the places to work on when/if you update it but getting more weapons and an upgrade system would be nice. Some music to fit the tone is a must, and good sound effects for shooting the guns and hitting the enemies.
Honestly you don't need to add much to this game, the whale design is just that compelling and a real selling point for this game.