Joker's never been terribly deep as a character no matter what they've tried to do with him. He can be whatever the story really needs him to be, and the best summary of his past is probably his 'multiple choice' quote as a handy wink wink to the audience. That being said, if there has to be a film about him, this is pretty as it goes.
Actually, it being divorced from the DCCU works best in its favor, because you don't have Supervillain Joker to compare him to. The character's backstory - at least in my opinion - has always worked best as Jack Napier: sad, failed comedian whose best shot at regaining control of his life is - ironically - a conscience rejection of control and inhibition that he only realizes after he's lost everything. It's endearing, even with regards to who it's about.
tl;dr Movie looks neat. Got a good feeling about it, I'll give this a shot.