In Germany, flatbread with a topping seems to have been a mostly Alemannian, Swabian and Franconian thing. A thin crust made from wheat flour and sourdough. Instead of tomato sauce, the base sauce will be sour cream-based.
The simplest set of toppings will probably be thinly sliced onions and/or sautéed leek.
Grated gouda- or gruyere-style cheese will soon become a popular option, whereas the traditional lardons will propably be replaced be other meat. "Schwarzwälder Fladen" might have slices of Black Forest smoked ham, other styles might have Plockwurst or a similar salami-like suasage (but probably not Bratwurst). Apparently, in Alsace, "Forester's style" means no lardons but mushrooms.
Additionally, the sweet version with sourcream, apple slices, sugar and cunnamon plus optional apple brandy will be popular too and propably be the ancestor of other Dessertfladen: In Germany, French crepes are usually offered with sugar, chocolate sauce, almonds, bananas, curacao liqueur, these sound plausible for a "sweet pizza", too.