There are so many good ones beyond the classic sauce, mozzarella, and pepperoni combo. Cooked and crumbled Italian sausage, sliced red onion, whole roasted garlic cloves...all good. My all-time favorite is 'nduja (if you can find!). And think about what leftover cooked meats and vegetables you have on hand. Pre-cooked and sliced meatballs are key in meatball pizza, and chopped up chicken pairs well with BBQ sauce and cheddar.