You could use leftover chicken or steak. My tip would be that you don't use a meat with a competing flavor or marinade. Check out my Cilantro Lime Chicken or Citrus Skirt Steak for some alternative meat options. That would be no good. Stick with something in the Mexican spice family, or neutral like garlic or pepper. Another option for a vegetarian twist would be using fajita veggies or cooked tofu.