PREHEAT barbecue to medium. Combine 1 ½ tbsp oil, chili powder, cumin, coriander and 1/4 tsp salt in a bowl. Add chicken, turning to coat. Season with pepper to taste. Toss peppers and onion with remaining oil and 1/4 tsp salt in a bowl.
OIL grill. Grill chicken, lid closed, flipping halfway, until cooked, about 20 min. Grill vegetables, flipping halfway, until tender and slightly charred, about 10 min for peppers and 20 min for onion.
ZEST and juice lime into a bowl. Add avocados and remaining salt. Mash with a fork until fairly smooth.
WRAP tortillas in foil and place on grill to warm, 2 min. Slice peppers. Transfer chicken to cutting board, let rest 5 min, then slice thinly.
SPREAD avocado mixture on tortillas. Top with vegetables and chicken. Serve with cilantro.