Sometimes, I like to experiment with the ingredients to use as toppings for risotto.
In order to avoid bad results, what should I consider when I choose the ingredients to use as toppings for risotto?
So risotto is pretty much a blank canvas, much like say rice or pasta.
So your first concern is simply to pick ingredients that go with each other. For example: tomato & corn risotto, good. Tomato and chocolate risotto: bad.
Second, you may want to choose a broth, cheese, and optional wine that goes well with your choice of main ingredients.
Third, because risotto is Italian in origin, the most certain combinations will be those classic Mediterranean flavors. You aren't going to go wrong with say asparagus and olives, or artichokes, peas and lemon. When you start making Thai risottos, you are farther out on a limb.
Finally, much of the pleasure of a risotto is in the rice itself, when properly prepared. That pleasure is fairly delicate in nature, so for maximum enjoyment you should choose reasonably delicate ingredients and/or use them in relatively small quantities.
Do you actually mean "toppings" as in, things to put on top of a plain risotto? I usually think of things to add to (mix in) a risotto. In that case (probably not answering your question), the important thing is to make sure your other ingredients are already cooked and then add them once the risotto is done. Otherwise, your risotto will not cook well.
Like Michael said, risotto is a blank canvas. It can just be a side on the plate (a tastier/fancy substitute for plain rice), or it can be the star, or even the entire dish, depending on what is mixed in it. An excellent side of risotto could contain savory vegetables like grilled asparagus and mushrooms, and you could accompany it with a chunk of protein of your choice. I've also had a seafood risotto as a main dish (containing various cooked shellfish) which had vegetables as an accompaniment. Whatever role you want the risotto to play in your dish will help determine what to put it, or accompany with, your risotto.