Preheat your grill or grill pan over medium-high heat.
In a small bowl, mix together the olive oil, chili powder, cumin, garlic powder, paprika, salt, and pepper to make a marinade.
Coat the chicken breasts with the marinade, ensuring they are well covered. Allow to marinate for at least 10 minutes.
Grill the chicken for about 6-7 minutes per side, or until fully cooked (internal temperature should reach 165°F). Once cooked, remove from the grill and let it rest for a few minutes before slicing it into strips.
On the grill, add the sliced bell peppers and onions. Grill for about 3-5 minutes until they soften and have grill marks.
In a large mixing bowl, combine the chopped romaine, grilled chicken strips, grilled bell peppers and onions, cherry tomatoes, corn, and avocado.
Drizzle with lime juice, and toss gently to combine. Top with chopped cilantro, and add cheese or sour cream if desired.
Serve immediately on individual plates.