Five stars for the pure that I needed to kill time... and this game did. In fact, now I'm late...
Please do another - here are a couple of suggestions.
Superhero theme is good - but maybe have the appearance of the hero change over the course of the upgrades (starts of looking like an average joe, ends up beefy and tanned :D)
I agree with the idea of being able to develop a particular tree of powers - more upgrades are unlocked at a certain level of a particular upgrade etc.
Mid level miniboss, then a final boss at the end :D
Otherwise, well done! Loved it