I looked at this earilier, and thought that surely, someone would come up with better suggestions than mine, but that has not happened. So, here we go. Please keep in mind, that the first things you add to the burger are salt & pepper.
*Bleu cheeseburgers: put a pocket of blue cheese in the middle of the burger
*Pizza burger: Use half/half ground chuck and Italian sausage...Add finely chopped olives (black or green) and canned mushrooms if you wish. Top with provelone cheese and a sprinkling of Parmesan.
*Steak burger: Use ground chuck or ground sirloin...add 1-2 tbl Worchester Sauce and/or steak sauce of your choice...Add chopped onions and mushrooms...pat out to size of fillet, wrap a piece of thick-sliced bacon around it, secure with a toothpick and grill or broil. Or cook onions and mushrooms separately in butter, using the same seasonings that you used in your steak. Use as a sauce for your steak.
*Chili burger: Season ground chuck with chili powder and cumin. Drain a can of Bush's chili beans and add to meat. Moisten with as much of the liquid from the chili beans as needed. Add sauteed chopped onions and peppers, if you wish. Top with Monteray Jack, Colby, and/or cheddar cheese of your choice.
If you want to get more vegies into your burgers, try adding grated carrots. I slip them in to all kinds of things. They are very sweet, and basically undetectable when mixed in with the meat.