For serving: sliced avocado, plain Greek yogurt or sour cream, chopped fresh cilantro, shredded cheese
Place the chicken thighs in the bottom of a 5 quart or larger slow cooker. Sprinkle with salt and pepper.
To the slow cooker, add the garlic, onion, diced chipotle peppers, adobo sauce, diced tomatoes, cumin, coriander, green chilis, drained black beans, and drained Mexicorn. Pour 3 cups of the chicken stock over the top. Cover and slow cook on high for 2 hours or low for 6 hours, until the chicken is cooked through.
With a fork, remove the chicken thighs from the soup, then shred. Return the chicken to the soup, and add additional chicken stock to the slow cooker if it seems too thick. Recover the slow cooker and cook on high to rewarm for a few minutes as needed. Serve hot with desired toppings.