Very neat and fun game! Got preatty addicted for a while :D !
I would suggest following improvements regarding the scope this project:
   - tweak UI a bit for more responsiveness
   - (maybe) more upgrades
   - make the runs quicker but more difficult (8-10 waves instead of 15)-> so decision making is more impactful
   - (personal, but...) I would change the way you can upgrade upon existing weapons, because in the long term it does not feel like a big improvement upgrading existing weapons
   - and may biggest recommendation is to make the player more involved in the game play, because it's boring to walk very long distances and having nothing to care about but just to upgrade your weapons (in relation with point 3); I would like just more actions (manual shooting) for the player regardless if movement or skills/specials

I really think you have a cool game with a solid gameplay-loop for its scope, but you can still enhance quite a bit on this game and make it even better ;). Nevertheless I wish you a great continuation for your gamedev-projects!