Worcester sauce, grainy mustard, garlic, chilli and tomato ketchup are vital ingredients to homemade burgers. Not too much though as they'll make the mix too wet.
I often use an egg in mine and also bread crumbs to hold it together.
If you're feeling really adventurous, cut up a 1" x 1/2" piece of cheese of your liking and wrap your mince mix around it so the cheese is in the middle
Not really an answer to your question but a little hint I picked up some where I roast a whole garlic bulb in the oven (with all the skin etc on) let it cool peel it and mash it up add to your burger mix it adds a lovely mild taste to them .