Personally, I hate the taste of tomatoes and onions. I do, however enjoy cheese on flatbread with other stuff. I love za'atar (Lebanese flatbread), and pide Turkish flatbread (especially with cheese and sucuk) I enjoy lahmacun (another Turkish flatbread) even though it involves onion and tomato, because it's not overpowering and the texture is good. And of course I love pizza with pesto sauce or garlic and olive oil.
I've often thought that pizza with either chugochujang (Korean pepper paste mixed with vinegar and sesame seeds), mole, or salsa verde would be delicious, but I agree that barbecue sauce on pizza sounds terrible. On the other hand, I've always been skeptical of benefits of putting barbecue sauce on actual BBQ; it's too sweet, at least the tomato paste-based sauces (which fortunately taste nothing like tomato) which dominate the Mississippi valley and the parts of the US without native BBQ traditions.
On the other hand, I don't understand margherita pizza. The point of a white pizza to me is avoiding tomato. So why does the standard white pizza at every pizza parlor replace the tomato sauce with actual tomatoes?
While I have a long list of things that shouldn't go on my pizza (tomatoes, olives, and mushrooms top the list) there's nothing edible that I think absolutely shouldn't go on a pizza. In Georgia I had a flatbread that contained cheese, egg, and butter. And in Turkey I tried a pide with cheese, pistachios, and sugar. While these aren't quite pizzas, the principle is close enough. For the unhealthiness-to-flavor ratio I don't think either butter or sugar is a worthwhile pizza topping, but both dishes were still enjoyable. So at this point, I figure that if you want to put gummi bears or Chex mix or fried ox testicles on your pizza, more power to you.
