I have been playing Defender’s Quest: Valley of the Forgotten, which is pretty good. One unusual feature of its advancement system is a slider for how quickly you gain experience points and cash. Want to triple your rewards or stop them completely? You can do that. Want to remove the penalty for failure beyond “start over”? You can do that.
Defender’s Quest also classifies its achievements by difficulty. The hardest amount to “do everything perfectly in New Game++ with only one of each tower.” The in-game text goes on to note that New Game++ is really hard and that you probably want to turn on triple xp.