The Joker.
Gone.
Yeah I see, maybe something like every enemy that you kill turns into a gold statue that either stays there or falls of the screen. Or you could unlock a gold costume to wear.
I just think that a gold statue and/on the title screen or selecting screens is a bit underwhelming considering how hard you might have to work for it, plus you won't even see it most of the time as you will be playing the game not staring at those screens.