I would say pepperoni, fresh sausage, white button mushrooms (or maybe cremini or portobello if you have to impress), and onion (sweet). Pepperoni is on the list because it is the most popular topping in the U.S., so statistically that makes sense. The addition of the sausage, which also usually shows up high in the list of favorite toppings, satisfies the carnivores when used with pepperoni. If you expect more vegetarians than carnivores, then you could substitute green peppers for the sausage. The four choices should give you 16 different toppings combinations. Using none of these toppings would give you an additional option, a cheese pizza. Extra cheese would increase the total number of pizzas also, as will pizzas with split toppings (e.g., half this and half that).