(take it as read there is cheese in each of these omelettes - grated, parmesan or fetta)

mushrooms, sliced (loads of them) - this is my fave
grated or thinly sliced zucchini +/- onion
cooked potato +/- onion
tomato, capsicum, mushroom
chorizo, onion, tomato
and one I haven't made but reckon would be YUM - smoked salmon, onion, sprinkle of dill & capers