Celery: Not essential but I love the fresh crunch it adds, you could sub for finley diced red onions or spring onions.
Parsley stalks: Instead of the parsley leaves i like to use the stalks in this recipe as they are crunch! You can substitute for chives or coriander/cilantro stalks too.
Dijon mustard: Any mild mustard
Cottage cheese: I use cottage cheese in my egg salad recipe instead of mayonnaise as it adds protein. This can be substituted for greek yogurt. If using Greek yogurt taste your egg salad mixture before you add the lemon juice as you may not need it as yogurt is already quite tangy.
Bacon: My kids love it when I mix crispy bacon pieces into the egg salad, but it is not essential.
Sliced bread: For me, when it comes to an egg salad sandwich, soft white bread is the best!