There are some rumors going around and evenResident Evil producer Masachika Kawata hints that if the game sells, DLC could be possible He says:

"At the moment no, we haven't got any plans," Kawata said. "If the users want that we will endeavour to make it happen. We're always monitoring the comments and feedback from users."

so that seems promissing and since i mean its Resident Evil that alone ensures some good sales so lets just sit tight and hope new characters make it to the Mercenaries 3d.