We know that the third movie is connected with Batman Begins and The Dark Knight and that Tom Hardy is Bane and Anne Hathaway is Catwoman (Selina Kyle).
We know too that Joseph Gordon Levitt and Marion Cottilard have been rumoured to be Black Mask (Roman Sionis) and Talia al Ghul. But how could they be introduced in the Nolan's storyline? And who can be in TDKR?
Joseph Gordon Levitt's possible characters
Black Mask (Roman Sionis)
It's rumored by several entertainment news websites that Joseph has been approached to portray Black Mask (Roman Sionis) in The Dark Knight Rises. Roman Sionis could appears as the newest mobster in Gotham City. He could fight with Bane by the Gotham City's underworld like Killer Croc in Knightfalls or the Black Mask could aid Bane and the League of Shadows like Carmine Falcone in Batman Begins. In the comic books, Roman Sionis was killed by Selina Kyle.
Deadshot
A old enemy (First Appearance: Batman #59 June, 1950) that could be introduced easy in Nolan's dark and realistic universe. Deadshot will a mercenary hired by the League of Shadows to framed Bruce Wayne for murder (like David Cain in "Bruce Wayne: Murderer?") or assist Bane.
Catman
→ He was a mutual enemy of the Batman and Catwoman.
→ He works alongside of Bane in the Secret Six.
→ He is a old enemy of the Batman (First Appearance: Detective Comics #311 January, 1963).
Jeremiah Arkham
Jeremiah Arkham could be introduced in Nolan's storyline. It's rumored that Arkham Asylum will appears in TDKR. So Joseph could play the Doctor Jeremiah Arkham.
The Reaper
One way to connect with TDKR and Batman Begins, it would introduced The Reaper (Joe Chill Jr.), like a mixture of Holiday (Alberto Falcone) and Joe Chill Jr.
Sasha Bordeaux/Talia al Ghul (Marion Cotillard)
Marion Cotillard was rumored that she has been approached to portray Talia al Ghul in The Dark Knight Rises. Talia could be shown as the new secret leader of League of Shadows and she could be first shown as Sasha Bordeaux (Like Ra's al ghul that showed himself as Henri Ducard in Batman Begins). She could infiltrate in the Waynetech as Bruce Wayne's bodyguard after the Bane's attack (or Black Mask's attack) in the Wayne Manor's "inauguration party". She could be hired by Alfred and Lucius Fox to protect Bruce against his wishes. Wayne tries to avoid her, but she keeps doing her job and eventually falls in love with him even while Bruce Wayne has a relationship with Selina Kyle.
Bane (Tom Hardy)
Bane could be shown as a mixture of Ubu (Ra's al ghul's bodyguard), and as the newest criminal mastermind. Bane could terrorize Gotham City with his ambition to destroy Batman and become the ruler of Gotham's criminal underworld. Also he could be aided by the League of Shadows (and Black Mask).
Selina Kyle/Catwoman (Anne Hathaway)
Selina Kyle could be shown as a charity fundraiser, in which she meets Bruce Wayne (Like in The Batman) in the Wayne Manor's inauguration party. As Catwoman, she could be a expert burglar hired by Bane (or Roman Sionis) to rob the Waynetech's industrial secrets, but she falls in love with Bruce.