Heh, haven't posted here in ages. Figured I'd remedy that and throw in my two cents for the casting suggestions.
Mark Hamill as The Joker. Despite his look not being perfect, he has the voice, and the attitude, as perfectly displayed in the cartoon and even more so in Batman Beyond: Return of the Joker. However, if the producers wanted to go with a more unknown person, or something new, then I would suggest...
Zack Ward, a relatively unknown actor, who some people may remember from Resident Evil Apocalypse, or from the tv comedy Titus. The above pic was from a dream sequence in Freddy vs Jason, where he showed a rather maniacal grin and laugh - despite the latter being dubbed over by Robert Englund, as Zack was Freddy in disguise - and I could see him in the part.
Hudson Leick as Harley Quinn. Anyone who has seen her as Callisto on Xena should know that she was rather deranged in many episodes, and I could definetely see her playing this role. She can play crazy/unstable, plus she's got the smile.
Tricia Helfer as Catwoman. She can be both seductive and a badass fighter - Battlestar Gallactica enough proof of that - she's a good actress, has green eyes - as Selina Kyle does - without the aid of contacts. I know alot of people have been saying brunette people for Catwoman, but since the character was blonde both in the cartoon and Batman Returns, I figured she should be blonde in this too.
Natassia Malthe as Talia. I don't have much to say for her, except that she's dark, exotic, and the only person with Talia's look that I could see as Liam Neeson's daughter.
Liev Schreiber as Two Face. Very good actor, and like many people here I could see him handling the part very well.
Thomas Krestchmann as Mr. Freeze. He looks alright bald, looks closer to the age of the character on the cartoon, is a capable actor, plus he has the accent and can come off as a crazed scientist.
I also think Crispin Glover for The Riddler would work, as would Bob Hoskins for Penguin. I would have posted their images, but it said the above pics was the limit, so, yeah