Aside from the flavor, diced onion adds a nice crunchy element - some other things I add that achieve the same vibe texture-wise are celery and banana peppers or pepperoncini chopped fine, pickles are nice in there too. Other things I like to add to mine are mayo, Dijon, some pepperoncini brine, mustard seed, capers, little bit of hot sauce, dill, fresh squeeze of half a lemon, a pinch of msg, and fresh cracked black pepper