Burgers are rich. They need contrast. Ketchup is fine, but pickles do more. Gherkins bring crunch and acidity. Pickled onions add sharpness. Chilli piccalilli adds mustard tang and heat.
Try a burger with gherkins plus a spoon of chilli piccalilli. Or use sweet balsamic onions for a darker, sticky finish. Keep it simple. One main condiment, one crunchy element and you are sorted.
Better burgers are usually about balance, not more toppings.