Hmm, I noticed something about this outfit earlier, it seems to draw a lot off his tattoos. The batsymbols are the same as the one tattooed on his bicep with the blade going through it, the stars on his shorts and the "haha special" logo are the same as the one tattooed on his face, and the boot has the same smile as his hand tat. Maybe this outfit had more significance to Leto's joker than was shown in the movie?
And also was there ever a reason for why he has boxing shorts on? Did he used to be a boxer? That would be a cool backstory that allows him to go toe to toe with batman in a later film