To me, the voice reminded me a bit of the voice for Scarface from TAS. Is it really Mark Hamill? And in the course of the game, does he fight through his classic Rogue's Galley that are usually imprisoned in Arkham?
I only know of the Joker and Harley as confirmed - though in the one teaser trailer I thought I saw Killer Croc.
Mister Freeze (Unconfirmed - but a voice actor is listed)
Scarecrow (Unconfirmed - but a voice actor is listed)