Preheat a skillet over medium heat and cook the bacon until crispy. Remove and drain on paper towels.
Season the chicken breasts lightly with salt and pepper.
In the same skillet, cook chicken breasts over medium heat until golden and cooked through, about 6-7 minutes per side.
Brush chicken breasts with barbecue sauce and top each with two slices of Swiss cheese. Cover the skillet briefly to melt the cheese, about 1-2 minutes.
Lightly toast the hoagie rolls in another pan or oven until just crisp.
Spread Ranch dressing on the inside of each hoagie roll.
Place the cheesy barbecue chicken on the bottom half of each roll.
Top each chicken breast with crispy bacon slices and diced avocado.
Close the sandwiches, slice in half if desired, and serve immediately.