This has been discussed before, but I'll say again that a life bar would totally undermine the game's core appeal. While the game does reward skill, chaos is the order of the day. There's a certain tension in not seeing your life bar. I also don't want the screen cluttered with them.
I wouldn't mind some kind of season system, though. It sounds fun.
What's going to keep me interested in the long term, though, is new maps. Even if they never came out with new skins or characters again.