If you hadn't mentioned casseroles, I would have said tortilla soup. It's what I always do with my leftover chicken and turkey (the meat, broth, corn, chopped onion and a package of taco seasoning). No tortilla chips, of course.
But for casseroles -- how about adding Ro-Tel and FF cheese? Or bake it with brown rice and vegetables?
I have to stop thinking about it though; I'm making myself hungry.
Good luck and be sure to let us know what you did! I need new ideas!