Gils and Darkstream,
When I went through my "authentic Neapolitan pizza phase", I found myself researching Naples pizza restaurant websites (usually in Italian) and looking at pizza menus to get an idea of the diversity of ingredients and toppings used on their pizzas. As you may know, the Verace Pizza Napoletana Association (VPN), the Italian "pizza-nazis", officially certified four Neapolitan pizzas:
· Marinara (Napoletana): Tomato, olive oil, oregano, and garlic.
· Margherita: Tomato, olive oil, grated Parmesan, and fior-di-latte or mozzarella.
· Ripieno (Calzone): Ricotta, fior-di-latte or mozzarella, olive oil, and salami.
· Formaggio e Pomodoro: Tomato, olive oil, and grated Parmesan
(Basil is also permitted on all of these pizzas, and variations are recognized if they are "informed by the Neapolitan tradition of pizzas and are not in contrast with the rules of gastronomy, with judgment reserved to the Association's committee".)
In May of this year, the Italian Ministry of Agricultural Politics in Italy proposed a new set of rules governing virtually all aspects of Neapolitan pizza production, from dough ingredients, to toppings, to processing and baking techniques. These rules were even more restrictive than those used by the VPN (but limited to Italy). The ministry's rules authorize the following pizzas:
Marinara (with garlic and oregano, tomatoes, and extra-virgin olive oil distributed in a spiral pattern starting from the center of the pizza), Margherita (with tomatoes, STG mozzarella cheese or fior di latte cheese from the southern Appennine mountains, fresh basil, and extra-virgin olive oil distributed in a spiral pattern), and Extra Margherita (with tomatoes, mozzarella di bufala from the Campania region, fresh basil, and extra-virgin olive oil distributed in a spiral pattern). (The rules also get quite specific as to tomatoes: "peeled and crushed tomatoes or fresh tomatoes grown in the Mt. Vesuvius area", which strongly suggests the San Marzano tomatoes).
Obviously, pizza restaurants can't survive on just three of four types of Neapolitan pizzas, but those that do make these basic pizzas will offer many other kinds of pizzas that are also classically Neapolitan style pizzas (like Quattro Formaggi and Quattro Stagioni, etc.). They will use common, locally available ingredients such as anchovies, arugula, artichokes, asparagus, zucchini, potatoes, prosciutto, ham, green onions, yellow cherry-type tomatoes, cultivated and wild mushrooms, olives, capers, broccoli rabe, roasted vegetables, salami, and a wide variety of cheeses beyond the classic mozzarella cheeses, including Parmigiano-Reggiano, grana padano, fontina, gorgonzola, ricotta, and provolone (regular and smoked). Neapolitans are also fond of eggs on their pizzas.
My best advice for anyone interested in the classic Neapolitan toppings is to go to the VPN website, at
http://www.verapizzanapoletana.org/vpn/index.html, and look for the link to "Members". These are the pizza establishments around the world that have agreed to abide by the strict rules and regulations promulgated by the VPN. Links to some of the member websites are also provided. I would look first at Antica Pizzeria and La Pizza Fresca to get an idea as to classical Neapolitan toppings. I would also look at Naples 45 which, together with La Pizza Fresca, are the only restaurants in NYC to be certified by the VPN. The VPN members list doesn't provide a link to Naples 45, but the menu can be viewed by going to
http://www.menupages.com/restaurantdetails.asp?areaid=0&restaurantid=3343&neighborhoodid=0&cuisineid=0 and clicking on the menu icon. For those who are truly anal on this topic, they can go to the VPN members list and do Google searches on other listed U.S. pizza establishement, or some of the listed Neapolitan pizza establishments. In many cases, you will be able to find and review pizza menus. Doing this will provide a treasure trove of toppings possibilities.
Gils, with the internet, you no longer have to go anywhere to get answers to your questions, even those as arcane as regional Neapolitan pizza toppings

. You can do it from the comfort of your armchair.
Peter
EDIT (1/9/2020): For a replacement link for the above inoperative link, see
https://www.pizzanapoletana.org/en/