I agree, no sexy, sassy, well known actress could pull off catwoman. They would need a very, very, special kinda - and I cannot think of any big named actresses that could nail it. Cat woman has a sort of mystique and class that is just whored down by modern big name actresses.
I very much disagree that the riddler is another joker. As an avid and long time fan of batman, I remember the joker being much more menasing and explosively insane. I think Nolan had the closest ideal joker I have seen in a long time. I think too many people look to the animated series joker, when really, they should remember the rand rocket launcher which ... well, sholdn't spoil that for people who go back and read. Either way, the Joker, is an Evil son of a bitch, as has been stated by Gotham's finest. I feel that the other incarnations of the joker really hybrid the Joker into more of the fun loving Riddler.
In short, the joker is demented-evil-clown, the riddler is witty, compulsive, and fun loving in what he does. I think (SPOILER) the guy that finds out Batman's identity in the movie will make a bigger appearance in the next film, and at first, I thought he was going to pop in as a riddler-esque character, just based on his actions and expressions.
Therefore, I really do not see true joker and true riddler as being too similar by any right. Aside form the fact that they thoroughly like what they do, when they do it. I think Nolan's riddler will be the next villain, because I feel that he is trying to bring the batman back to the darkness which it should have shared with its roots.
I agree with your last thought too, about bringing more detective into the mix. So far, the detective side has been following the bond style mystery line and not that of the holmes. That would be a cool critique to take on the third.