I’d do a mushroom and walnut burger. Throw the traditional lettuce, tomato and onions on that you get with the Whopper, that would be a good sandwich
I think the original Burger King vegi burger had water chestnuts in it too. It was really good. It came with lettuce, onions, tomato, and the Whopper sauce. It was never much promoted and it often wasn’t advertised on the menu, but you could always order it.