In a medium saucepan, combine the cooked shredded chicken and buffalo sauce over medium heat. Stir until the chicken is heated through and well coated with the sauce, about 5 minutes.
While the chicken is heating, prepare the brown rice according to package instructions if not already cooked.
In serving bowls, layer the cooked brown rice as the base.
Top the rice with the buffalo chicken mixture, followed by black beans, corn, diced tomatoes, and avocado slices.
Sprinkle shredded cheddar cheese and chopped green onions on top.
Drizzle with ranch dressing if desired. Serve immediately.