Cook the rice: Bring 8 cups of water to a boil in a large saucepan (3-quart or larger). Stir in the rinsed brown rice and reduce heat to maintain a gentle boil. Cook uncovered for 30 minutes. Drain well, return rice to the hot empty pot, cover with the lid, and let steam for 10 minutes before serving.

Roast the sweet potatoes: Preheat oven to 425 degrees F. Toss sweet potato cubes with olive oil, chili powder, salt, and pepper in a large bowl until evenly coated. Spread in a single layer on a parchment-lined baking sheet. Bake for 18 to 22 minutes, flipping halfway, until edges are caramelized and centers are fork-tender.

Assemble the bowls: Divide chopped romaine between 4 serving bowls. Add brown rice, roasted sweet potatoes, black beans, corn, and diced tomato or pico de gallo to each bowl. Top with guacamole or sliced avocado. Serve with fresh cilantro, a squeeze of lime, and salsa if desired.