Hey now, Scarecrow is still out there. While it might be a bit of a retread if Nolan used him again for the whole film, he could still be in play.
Me, I like the idea of The Penguin. Never the most effective Batman villain, perhaps, I think he would work great not as a freakish character or a weirdo with a penchant for funny umbrellas but, rather, as a vicious mob boss with an unusual nickname (perhaps because of his penchant for wearing exquisitely tailored tuxedoes?). There was a rumor going around right after Dark Knight came out that Phillip Seymour Hoffman was in consideration for such a role, and I really could see him playing a great Penguin in this vein.
Neil Patrick Harris as The Riddler (another, more recent, rumor) could be very cool too (David Tennant would work as well). I doubt that, if Nolan uses the Riddler, he would play him too much like The Joker.
I don’t, however, want to see Catwoman. I just don’t find her to be that interesting a character. The Huntress could be interesting, particularly if the end of Dark Knight is any indication and Batman will be perceived as the villain in the next film. Huntress could be played as a vigilante trying to take his place AND hunt him down at the same time. Not sure who I would like to see in a role like that, though I keep thinking of Scarlet Johanson (maybe my memories of Iron Man 2 are still strong).
Vin Diesel as Bane…hmmm…that could have possibilities…