http://uk.movies.ign.com/articles/112/1128133p1.html
A good article.
Deathstroke would be interesting because he is awesome.
Of course, sticking with Nolan's theme all of these villains would have to be quite psychological and dark. According to IGN, Catwoman is likely to make an appearance, though it is very unlikely she would be the primary antagonist, methinks.
Although Riddler is still my favourite, I think characters such as Hush or Black Mask would work well. Croc is a bit too 'unrealistic', as is Clayface, unless they drastically modified them. Likewise with Poison Ivy - her whole background story is quite 'unrealistic' and even though
Plinglebob said:
One idea I read was have Poison Ivy as someone who uses plant toxins in eco-terrorist attacks.
this is a good idea, but it is similar to Ra's Al Ghul's plan in the first film.
Similarly, old Freezey would be quite hard to pull off. If I remember correctly, I recall reading somewhere that Nolan stated he wouldn't ever use him in a film.