Obviously the Joker has something to gain......with all the mythology backing the Black Mask and his false face society the two fit together quite flawlessly.
As far as Talia goes....go back and watch Begins when Ras speaks of how the League of Shadows has penetrated every level of establishment in Gotham and thats where this fits in....but instead of the League wanting to kill Batman the want to recruit him to lead their team in replace of Ras. In order to defeat Black Mask he needs the help of The League but at the moment he realizes it...All hell breaks loose with Mr.J getting loose.
I need some more ideas for version 8 but I cant let go of the story's best arc The Joker was part of the False Face Society Black Mask was once in the League of Shadows and like Black Mask, Joker went rogue.
I like Bruce Wayne trying now to fight with out the mask, using Wayne Enterprises and then to some extent its destroyed right in front of him first by Black Masks intel and destroying the name of Wayne Enterprises, then physically by the Joker. At the end of the story Bruce realizes he as well as Gotham City need Batman to balance the order against the chaos and corrupt.
I understand where your ideas are coming from and believe me they are great, but I myself have issues with a few things.
The mythology your talking about only exsists on the comics. In order to adapt your idea to what Nolan has already done Black Mask and his false face society would have to have been around before Batman Begins. I just don't see that working because it's almost like you're going backwards in storytelling.
I'm very familar with Batman Begins and I like Talia, but I think your ideas here are too close to what came before. Give me something new, give me something fresh.
As far the Joker goes, I don't want to know "The who, the how, and the why". I just want the mystery. You would also have to recast the Joker and I don't see that happening either.
I do think Bruce Wayne and Batman could battle Black Mask though.
In closing, as I said I really like your ideas, but... I don't think they fit with where the story NEEDS to go after the events of TDK. If you're just doing this for fun then by all means take all the liberty's you want.
Now, I'd like to get your take on an idea of mine. This is something I came up with while reading your stuff. Here goes...
Draft One
The story would be set a year after the events of The Dark Knight, giving Roman Sionis plenty of time to come and "setup shop" in Gotham. He could also use his dual identity to the fullest. As Sionis he would be everything that Bruce Wayne is on the surface, a philanthropist, who in the public eye is totally clean. As Black Mask he would control the largest organized crime family in the history of Gotham. (We need escalation here, a mob boss and a "freak" in one.) With most of the mob left in chaos the underworld would be his for the taking. Sionis still battles Wayne Enterprises and Bruce gets the feeling that he's dirty, but has to prove it as Batman.
There also has to be a rouge element, which is where Catwoman comes into play. Her background would have to be changed a little to fit the story, but her main motivation is to kill Black Mask. He killed someone close to her and she's one of the few people alive who knows that Sionis and Black Mask are the same person. She strikes at him in the underworld as well as publicly, which puts her on the outcast list. The police want her because he's tried to kill an important business man, Batman wants her because she's interfering with his crusade against injustice.
The final player in the game would be the Penguin. Oswald Chesterfield Cobblepot, flashy nightclub owner to the public and seller of arms and information to "other parties". He sells weapons to the False Face Society to help take over Gotham, while selling information about thier leader to the police and Catwoman. His motivation is to reap the largest benefits, while secretly hoping they wipe each other out so he can step in and take control of Gotham. To make is character a little more real the nickname of "The Penguin" could be a joke in the underworld. He wouldn't even need to be called The Penguin. Black Mask sends his men to see "The Bird" about weapons.
The final act of the film could play out with Black Mask holding Gotham at ransom with a nuke. Batman finds him, and with a little help from Catwoman, proves to the public who Sionis really is. Batman redeems himself in the public eye, but Catwoman is forced to reveal her identity and answer for trying to murder Sionis.
I think this this flows well with Nolan's crime epic, gives us a nice duality theme and still manages to deliver three NEW characters with solid motivation. Let me know what you think.