Based on your liking the topping can vary from the whole range of vegetables, cheese, eggs, nori, mushrooms and so on.
Even thou one could add many different ingredients, it is still important to keep track of the flavours, trying to stick with one or two dominant flavours, and arrange the rest to fit it.
In this case the dominant flavour is the meat. Second to it are the shitake mushrooms, the rest just completes the palette ;).