Real Talk: Im torned on this. On one hand, a film like this is absolutely unnecessary. The Joker works far better when hes a mysterious character and any kind of origin to explain his madness would sully him. This could probably show us what he thinks was his origin a la The Killing Joke but given the director of this thought a film where a main character yells out Doctor F****t was comedy gold, I doubt theyll go that direction. And a good cast is never always an indicator for quality.
But I do like the idea of a separate set of films that stand alone from the correct DC cinematic universe. It could potentially allow for more creative leeway for filmmakers who arent so concerned with connecting to a larger mythos and simply want to create their own worlds and stories that could be seen as their own. There are certain characters that couldnt work in the current universe established, like The Spectre, who could really use some exposure. Heck, as much as shes been overexposed, I wouldnt even mind a stand-alone Catwoman film since theres more to do with her than with the Joker, like in When in Rome. After her last stand alone, theres no where to go but up.
So, yeah. The concept is cool but starting this off with a character that doesnt work as a stand alone lead for the most part, I dont know how to feel about this. I mean, all things considered, at least the Joker still exists in the current universe of DC films, whether we like that one or not. Venom, on the other hand, is a different situation entirely.