I actually think a Joker solo movie could be interesting.
I think it could be cool if they did one dealing with his origin, but do it in a trippy Fight Club style of filming. I'd have it start with him as a child, but throughout the movie have things happen that contradict what happens to him later in life. At one point in the movie we see his father scarring him as a boy, later in the movie we see him older with no scars, being a struggling comic with a wife, and scarring himself. Then we see him working for the mob, and getting a chelsea grin after a botched operation.
I'd also incorporate the kind of subliminal messaging they do in Fight Club, except have flashes of the Joker, or random flashes or horrible images.
There would be a general story that the movie follows, but as I said, events here and there would contradict each other. At the end of the movie, I'd have it flash to a scene of the Joker holding a knife to someone (any random person) and end it with him saying "...and that's how I got these scars..."
The whole point being that the movie is basically all being played out in the Joker's head, which is why it would have a frantic, odd filming style to it. With quick cuts and the flashes of images that seem unconnected to the story, and at the same time having different events in the story contradict themselves.
Basically, the movie would offer multiple explanations to how he becomes the Joker. It would follow a semblance of linear story-telling (starting with him as a boy and ending with him as an adult) but at different points in the film he's dipicted as experiencing different things and acting in completely different ways.
For example; one scene of him as a child would show him as an innocent boy being abused by his father, later, we see him acting like a horrible little sociopath, lighting a cat on fire just to amuse himself.
At the end of the movie, the audience wouldn't have a clear answer of what made the Joker how he is, but only be provided with possible explanations. It would be carrying on the theme of TKJ basically, and providing us with an insight of how the Joker remembers his past.
....but to make it clear, NONE of this would involve Brett Ratner.