Feta. Sure, pizza tends to be already covered in cheese, but this is different cheese, and that is a good thing. I think it goes well with some of my other faves, such as green olives, pineapple, banana peppers.
Salami. It's like pepperoni, but different.
Alfredo sauce instead of tomato sauce, together with a few toppings like chicken and red onion, is heavenly, and doesn't seem to cause heartburn.