Prepare the chicken by shredding it into bite-sized pieces. If using raw chicken, cook it thoroughly until it reaches an internal temperature of 165°F (75°C) before shredding.
Make the enchilada sauce by combining diced tomatoes, chili powder, cumin, and garlic powder in a saucepan. Simmer over medium heat for 10 minutes.
In a large mixing bowl, combine the shredded chicken, black beans, corn, and enchilada sauce. Mix well until evenly coated.
Spread the cooked brown rice evenly across the bottom of a 9x13 inch baking dish.
Pour the chicken mixture over the rice and sprinkle shredded cheddar cheese on top. Preheat the oven to 350°F (175°C) and bake for 20-25 minutes until the cheese is melted and bubbly.
Let cool for a few minutes, then top with diced avocado and chopped cilantro before serving.