Combine black beans, corn, tomatoes, onion, cilantro, serrano peppers, and garlic in a large bowl. Mix in 1/4 cup lime juice. Refrigerate mixture until flavors mingle, about 1 hour.
Place avocado in a container with remaining 1/4 cup lime juice, to prevent browning.
Place 5 tortillas on a microwave-safe plate and cover with a damp paper towel. Microwave until warm and pliable, about 30 seconds. Repeat with remaining tortillas. Scoop bean mixture into tortillas; garnish with avocado slices and feta cheese. Serve cold.