If you want to make your burger stand out from the pack, you don't have to lean on fillers or gimmicks. Instead, play around with the composition of the patty itself.
Mixing two kinds of meat together can add flavor complexity, improve the texture and mouthfeel, and create a memorable patty that won't dry out on the grill.
As long as you have a good ratio of meat to fat, which should be around 80 to 20, you can play around with using different meats to make a signature flavor.
Lean beef can end up dry if cooked too long, so try mixing in a bit of ground pork with your lean beef, which can have 50% fat or more with a fairly neutral flavor.
Adding a little ground veal can lighten the texture to different types of ground beef, and adding ground lamb to your burger will give it a more complex, gamey flavor.
If you're not sure how much of each variety of ground meat to mix together, start with half-and-half blends or use equal parts of each if you're using more than one type of meat.