I love the game. 2 suggestions:
1: With the defense system, perhaps adding a percentage for the damage you resist per being hit(Not very much maybe only 5% at 1000 Defense rating).
2: Either tone down how much it costs for everything(Due to growth rate) by 15%, or increase the rewards be get from playing-over levels again by 25%.
Online play would be amazing too. But one game you could incorporate for yours would be like in Doom Forge: You use auto attacks but tons of skills to start like 5. And just gain more and more including passive abilities and buffs. Yours is similar, just restricted skill count(May want to move it up from 6 to 10 if you want to add more skills into it and let people hotkey them by choice).