Casseroles are ideally suited to using up leftovers. So substitute or add depending on what your fridge has to offer. I like to bear in mind the living conditions of animals from which we use meat and thus choose to use meat from free range and organically raised animals. You may wish to do the same. All root vegetables like carrots, turnip, parsnip etc. would go well with this dish if browned at the beginning, as would most beans, squash and tomatoes.