Oatmeal, bread crubs, starch...?
Unless you enjoy meatloaf on a bun.
Talk to a professional chef and you want a ~30% fat / 70% protein ratio. With this ratio the patty is optimally juicy and flavorful, without the need for a binding agent like egg (which takes away from the "meat" taste). Ideally you grind your own meat or have the local butcher do it for you.