Suppose I'll give my two-cents on this subject...
I believe Liev Schrider would make a great Two-Face. He's a good actor and is capable of pulling off the darkness and tragedy of the character.
Jude Law would make a very good Riddler, as he can pull off the contemplative self-amusement the character possesses.
If Mr. Freeze is ever used, I would think Christopher Lloyd would accomplish a great deal as one of the most tragic characters, and could perform leaps and bounds over what Arnold did to the character (though I don't blame him as much as Schumacher and the script-writers...).
And finally, for my personal favorite, the Mad Hatter. As far as I know, this is a new name being thrown out for Hatter, but I could be wrong. Curtis Armstrong...famous for his role of Booger in "Revenge of the Nerds." He's mostly a second-rung comedy actor, but I think he could pull off the creepiness of the Hatter. He fairly looks the part, as well.
What do you guys think?