I loved this game, reminded me on Sonic!
I think that there could be a few more indicators to make sure the player knows that when the level starts over again that they have been upgraded and there are more coins out there to find.
I like the idea of being upgraded, would like to see maybe if the visually the character could start changing. Overall, with the amount of time we all had this is pretty great, I think you should polish it up into a full game!