It all depends on the CUT of meat. A good sirloin burger needs no extra additions except salt & pepper. But the cheaper stuff, like chuck, needs help and flavoring.
I add milk. If you think that sounds gross, remember milk comes from cows.
Sometimes I make meatloaf burgers. I add milk, shredded up pieces of bread, which soaks up the milk and keeps the burger juicy, oatmeal, S&P, soy sauce, garlic, onion bits.
I do turkey burger several ways:
Add a huge dollop of blue cheese dressing to each burger before cooking and mix in.
Or: add in a huge dollop of ranch dressing instead, plus grated Parmesan cheese and a teaspoon of melted butter to each burger.
Or: add a huge dollop of of Italian dressing in and mix.
Let marinate for 10 minutes or more.
These work best on the George Forman grill.