Meat in bread indeed is not the German part, for a German Döner veggies and sauce aren’t optional, even when served on a plate. Default is Tsatsiki – not even Cacik, but the Greek stuff, without mint, dill, or extra water. Cucumber, tomato, onions, and some sort of cabbage as veggies, as well as the option of with Scharf, implemented via (usually pickled) Jalapenos and/or Sambal Oelek. There’s various things to the whole thing you see in neither German or Turkish cuisine, it is true fusion food, wouldn’t be possible without the different cuisines meeting.

As for the word, no, this is like the Italians trying to regulate what “Pizza” means instead of, rightly, regulating what “Pizza Neapolitana” means. If Swedes want to put pineapple in their Döner then Germans are going to join in with Turks calling it a crime against food but we’re also not going to stop them.

I’ve heard all kinds of crazy stuff from outside Germany, like using ketchup or mayonnaise, can’t even decide which is worse they’re both atrocious choices. There’s exactly one valid reason why you would use a sauce that’s not yoghurt-based, and that’s because you’re making a vegan variant – which would then imitate a yoghurt-based sauce (Vegan is not at all common but veggie options aren’t rare, usually replacing meat with falafel otherwise the same concept).