Combine black beans, sweetcorn, tomatoes, onion, coriander, chilli peppers and garlic in a large bowl. Mix in 4 tablespoons lime juice. Refrigerate mixture until flavours blend, about 1 hour.
Place avocado in a container with remaining lime juice (to prevent browning).
Place 5 tortillas on a microwave-safe plate and cover with a piece of damp kitchen roll. Microwave until warm and pliable, about 30 seconds. Repeat with remaining tortillas. Scoop bean mixture into tortillas; garnish with avocado slices and feta cheese.