I know we all wish and hope the next xmod will be {insert fantasy xmod here}, but it comes down to licensing. I figure RS went with the mustang cause of past Ford cars and success of them, not to mention Ford was probably more willing to give them the license to create a xmod on some of their models.
Course seeing how xmods are still on-going and some anticipated cars are coming out like the Camaro, Challenger, who knows what the next xmod in the line will be