Johnny Frost was as interesting and fleshed-out a character as he needed to be, what´s fresh about him is he´s neither a superhero nor a victim or a cop, he´s a thug who sees the Joker as his big break, he represents our dark fascination with The Joker, how there´s part of us that enjoys and even admires his mix of genius and perversion, but that ultimately has to realize how horrifying and ultimately hollow a man like him is at the core of it. Johnny is the naive criminal who thinks Joker is just the ultimate criminal mastermind, that even his crazy actions all part of his master plan, till he slowly but surely realized it isn´t. And there´s freshness in it being a take that´s more grounded, without much of the prankster/circus clown Joker, but keeping his perverted humor, cruelty, nihilism and emotional swings.
If you expect that after 70 years and thousands of stories something done with The Joker will be a groundbreaking reinvention of the character in comics, you´ll be disappointed every time. Pretty much every good Joker story done in the last 20 years or so basically takes on the motifs laid out by his first appearances, Joker´s Five Way Revenge, The Laughing Fish or The Killing Joke. Even in TDK, a big part of the success of this portrayal is it´s takes on Joker that we saw in comics, but not on film, such as the nihilistic "philosopher" or the unstoppable serial killer who´s constantly two steps ahead of the police and even Batman. It´s jazz, you can make great riffs of it, but the motif will always be the same.