This is clearly not vegan (cheese, egg, butter) but as formulated above uses no meat in the usual sense. It's similar to my favorite Korean dish bi bim bap, but the cheese and olive oil are of course dead giveaways ;) Carrots or peas I think would be good in it as well, as would bean sprouts, chopped avacado, slices of tomato, steamed squash ... I've made similar dishes out of various leftovers. For meat-eaters, bacon or chopped Kielbasa would turn this into (what I would consider) a well-rounded breakfast.