I personally would like to see Penguin as a main mob boss i Gotham after the aftermath of 'The Dark Knight'. Also it would be fantastic if they expand the theme of escalation that was set in the previous films with appearances of more freaks controlling the underworld and lurking in the shadows of Gotham. Characters like Ventriloquist/Scarface, Mad Hatter, reimagined version of Killer Crock could easily be present in the film, without explaining their backstories aside from their motives which define them, and Penguin could be their leader or the boss that took over after Maroni's death.
I am more than happy with the confirmation of Bane and Selina Kyle. It seems natural for ending a trilogy. Bane could be easily linked with the Leagues of Shadows, who would seek revenge, and the police of Gotham from which Batman is on the run. He seems a natural choice for a villain who would match Batman in physical combat.. but it would be even more perfect if the world of Gotham was expanded and made rich with characters like Penguin, Mad Hatter or Scarface, villains who can hardly carry a film by themselves, and would more than fit in the already established setting.