Stir in chicken broth, quinoa, enchilada sauce, green chiles, corn, black beans, cilantro, cumin and chili powder; season with salt and pepper, to taste. Cover and cook on low heat for 4-6 hours, or until liquid is reduced.
Remove chicken from the slow cooker and shred, using two forks.
Stir chicken into the slow cooker; top with cheeses. Cover and cook on low heat for an additional 10-20 minutes, or until the cheeses have melted.
Serve immediately, garnished with avocado and tomato, if desired.