I'll elect to draw on the rosters from both the cartoon and the recent comics:
Lex Luthor: Michael Rosenbaum (not going to happen, but I can dream)
Joker: Crispin Glover
Grodd: CGI, voice Brian Cox
Cheetah: Kate Hudson (a rich, entitled, spoiled girl-woman who automatically generates negative feedback from me, she'd be perfect)
Solomon Grundy: Adam Baldwin
Toyman: Carrot Top (come on, who wouldn't pay to see this guy get punched in the face repeatedly by Batman)
Brainiac: Jeremy Irons