A few suggestions:
Music, including a mute button
Change name of Blood to Health
Speed to Attack Speed
Power to Attack Power
Capitalization on Easier, Similar, or Harder.
I don't know if there is a save file yet, but if there isn't, you should make one.
The ability to choose your stat points in the beginning of the game
This is you and you ROCK!
This is the enemy and they SUCK!
You made some AWESOME upgrades!
YOU ARE THE BEST WINNER I'VE EVER SEEN!
CONGRATULATIONS FOR EVER