The Riddler is the most wanted villain for the sequel, according to a poll in another thread of these boards. Coincidentally, he has been linked to some form of social darwinism more than once here: in a popular idea among many posters, the Riddler should be adapted with a twist into a very probable storyline of Batman being hunted by the authorities, and that twist makes Edward Nygma/Nashton an FBI agent leading a task force team fully bent on identifying and catching the Batman, now Gotham's public enemy number 1. That version of Nygma should be all about his intellectual superiority and how the Law (his side of the battle) is entitled to do anything to get the work done. His obsession and frustration with aprehending the Batman leads him to the most unorthodox methods: creating a fake criminal persona called the Riddler, fashioned after Gotham's emergent criminal freak movement, and using him to place deathtraps with Riddles with the objective of luring the Batman out and defeating him. Eventually Nygma goes mad and fully embraces the Riddler persona, fixated with beating the Batman at any cost.
Another more traditional route for the Riddler, also discussed here, involves him leaving large deathtraps that put many people in danger, taunting the Batman into a game of wits. Here the social darwinism angle is also quite feasible, since the Riddler that knowledge is power and he is just gaining the power he deserves and cleansing Gotham in the process, letting those who can't solve his riddles die in his traps, weeding out the imbeciles and ignorants he considers societies biggest restraints.
In any case, the next villain should posit some kind of ideology, and in implementing that ideology, along with his personal goals, he should put Batman and all of Gotham in danger. That is, after all, what Nolan's series have consistently been about.