I've stuffed them with lots of different things - Mozzarella and Bacon (or Ham); Mushroom Sauce (can use any mushrooms); Peppers (can be stuffed) and Mozzarella (or other cheese); even Horseradish. Anything that isn't too mushy; compliments the beef (or other meat) and keeps it moist is great.
Chicken Burgers stuffed with Sage Stuffing or Turkey Burgers stuffed with Cranberry sauce and extra Cranberries are great alternatives for the ladies!!
Just go with the classics, lamb+mint; pork+apple; etc etc