Lightly spray inside of large slow cooker with cooking spray.
In a large slow cooker, combine the chicken, diced tomatoes (with juice), chicken broth, chili powder, cumin, and garlic powder. Make sure the chicken broth covers the chicken, adding more if needed. Cover and cook for 3 to 4 hours on the low setting.
Uncover and stir in the beans, brown rice, and corn. Cover and cook for another 3 to 4 hours on the low setting.
In the last hour of cooking, check the brown rice occasionally, stirring once or twice to make sure it cooks evenly and adding more chicken broth if the mixture seems dry. Cooking is done when the brown rice is tender.
Remove the chicken and place in a large bowl or cutting board. Use 2 forks to shred the chicken into bite-sized pieces. Transfer the chicken back to the slow cooker and mix.
Serve in individual bowls alongside optional toppings of choice.