Preheat a grill or grill pan to medium-high (about 450°F).
In a small bowl, whisk together 1 tsp chili powder, 1 tsp cumin, 3/4 tsp salt, 1/4 tsp black pepper, and 1/4 tsp sugar.
Brush the chicken with 1 Tbsp olive oil and season both sides with the spice mixture.
Grill the chicken 5–6 minutes per side, until the thickest part registers 165°F. Transfer to a cutting board, rest 5 minutes, then dice into small cubes.
If using fresh corn, brush with a little oil and grill over medium-high heat, turning every 3 minutes, until slightly charred (about 9 minutes total). Cut kernels from the cobs. If using frozen corn, set aside to add later.
Heat the remaining 1 Tbsp olive oil in a large pot over medium heat. Add the diced onion and sauté about 5 minutes, until softened.
Add the diced jalapeños and minced garlic and sauté 2 more minutes.
Pour in the chicken broth and the can of fire-roasted tomatoes. Stir in the remaining 1 tsp chili powder, 1 tsp cumin, 3/4 tsp sugar, and season with salt and pepper to taste.
Bring the soup to a light boil, then reduce heat to medium-low, cover, and simmer 15 minutes. If using frozen corn, add it during the last 5 minutes of simmering.
Add the grilled diced chicken, corn kernels, chopped cilantro, and lime juice to the pot; stir to combine and warm through.
Ladle soup into bowls and top each serving with diced avocado, shredded cheese, sour cream, and tortilla strips or chips as desired.