In a medium saucepan, combine the shredded chicken and buffalo sauce over medium heat. Stir until the chicken is well coated and heated through, 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, black beans, corn, diced tomatoes, and avocado slices.
Sprinkle shredded cheddar cheese and chopped green onions on top.
Drizzle ranch dressing over the bowls if desired. Serve immediately.