Step 1 Place a rack in center of oven; preheat to 425°. On a large baking sheet, toss chickpeas, 2 tablespoons oil, juice of 1/2 lime, and 1 teaspoon salt. Spread in an even layer.
Step 2 Roast chickpeas, stirring halfway through, until crispy, about 30 minutes. Add chili powder, toss to coat, and continue to roast until color slightly deepens, about 5 minutes more. Transfer to a plate and let cool.
Step 3 On same baking sheet, toss corn with 1 tablespoon oil. Roast until just starting to take on color, 8 to 10 minutes.
Step 4 Meanwhile, in a small bowl, toss onion, tomatoes, 1 tablespoon lime juice, and a pinch of salt. Let sit until ready to use.
Step 5 In a blender, blend yogurt, garlic powder, onion powder (if using), half of jalapeño, cilantro stems, half of cilantro leaves, one-quarter of avocado, 2 tablespoons oil, juice of 1 lime, and 1/2 teaspoon salt until smooth. Taste and adjust seasonings as needed.
Step 6 On a platter, combine tomato mixture, corn, remaining jalapeño, and half of remaining cilantro leaves; season with salt, if needed. Spread in an even layer. Top with chickpeas, remaining avocado, and a drizzle of dressing. Sprinkle with feta (if using), chives, and remaining cilantro leaves.