Depends on what meat I choose to grill.
Ground beef is usually just salt and pepper. I have tried a little bit of cinnamon to good effect; but, I really like the taste of beef in hamburgers.
Fish is usually just salt and lemon juice.
Ground turkey, or chicken, I put sage and a little olive oil into the mixture. Poultry makes a really dry ground meat and a little olive oil helps provide moisture.
Grilling onions I use butter, salt, pepper, and Worcestershire sauce. Peel the onion and slice an X on the end. Just wrap the onions, and ingredients, in foil and grill.