Perhaps adding more upgrades that are much more effective, along with a "Money Multiplier" upgrade(Limited). But giving us unlimited upgrades will add the addictive nature of the game for players to be extremely competative such as unlimited ammount for speed and jumping distance, but the rest are limited. People WOULD enjoy that competative element into it.
The game's concept is pretty good, but lacks some fundamental "Trainings" that you should be able to do to help train your stats. Maybe do training selects to increase your statistics, and then buy upgrades with cash you earn would be separate.
Overall, the game deserves around 3.5 Stars. I would like to see this game get improved.