Preheat oven to 375°F (190°C).
Mix ingredients: In a large mixing bowl, combine chicken, rice, black beans, diced tomatoes, corn, red and green bell peppers, onion, salsa, chili powder, cumin, garlic powder, salt, and pepper. Stir until everything is well mixed.
Assemble casserole: Spread the mixture evenly into a 9×13-inch baking dish.
Add cheese: Sprinkle the shredded cheese evenly over the top.
Bake: Place the casserole in the oven and bake uncovered for 20 to 25 minutes, or until the cheese is melted and bubbly.
Garnish and serve: Top with cilantro, avocado, or a dollop of Greek yogurt or sour cream before serving, if desired.