Most veggie patties are based on the soybean. Soy is incredibly versatile and can be morphed into all sorts of textures. Some newer varieties of veggieburgers are based on mushroom. I think these are even tastier than the soy versions.
The patty of a veggie burger can be made from vegetables, legumes, nuts, dairy, mushrooms, soy-derived textured vegetable protein (TVP), wheat gluten, eggs, or a combination of these. Most veggie burgers are now made from soy.
You could have variety of ingredients in a veggie burger. You could select vegetables from among the following - onions, capsicum, tomato and lettuce leaves. You can sprinkle some salt and pepper and add a layer of tomato sauce. if you prefer, you could ad mayonnaise too.