A villian of no real purpose isn't Nolan's style.
He doesn't do dumb plots that don't develope into the over all bigger picture. Ra's, Scarecrow & Joker were all bigger threats to Gotham before they were threats to Batman.
Nolan's Batman is about Batman's connection to protecting his parents dream, Gotham City. Too get to Batman, you have to go attack Gotham. Bane will be something more than an obsessed lunatic wanting to replace Batman.
But what kind of purpose does a drug king serve? We've already dealt with mobs in 2 movies. The Joker and Ras had greater ideals than drugs or money. Bane has to be above that or else it's dealing with the usual scum bags of the city. You mustn't be afraid to dream a little bigger darling.
I believe the point of that argument actually was about dreaming bigger. One very annoying thing about superhero stories is the tired "villain obsessed with hero" cliche. The original point was that Batman was somewhat incidental to certainly Ra's plans, and Joker was also out to create city-wide chaos - they were after destroying Gotham, which brought them into conflict with Batman. (Though I suppose it's arguable that Joker was after destroying Gotham for the express purpose of being in conflict with Batman.)
So the idea that Bane will likewise be after the destruction of the city is certainly feasible, and is, in fact, bigger than him just being interested in destroying Batman.
Given the "Hardy is not the main villain" rumors and the possibility of second female lead - I still think of Talia, seeking to complete her father's work (most likely at his behest since I'm pretty convinced Ra's is not dead in the Nolan-verse), using Bane. It might be somewhat more personal at this point - Ra's and Talia both (or just her on the off chance that Nolan skips the cardinal comic rule, no body, no death) out for Bruce's blood and his beloved city for the humiliation he visited upon Ra's and the thwarting of the original League of Shadows goal of carrying out their centuries-long mission of bringing balance back to society. This gives us a villain still seeking to wipe out Gotham (perhaps some version of the
Legacy storyline which includes Ra's, Talia, Bane and Henri Ducard) but with a very intimate conflict at its center.
Catwoman seems somewhat superfluous to this set up, but if they do a Bruce Wayne/ Selina Kyle romance, she could have more relevance.
Pure speculation of course, but I have long suspected Nolan would spring a surprise reveal of the seemingly immortal Ra's return. Should please the long-time Ra's fans, since it would cement him as the Batman's ultimate nemesis in the Nolan-verse.
