Hot Dogs; every part of the US has at least one really good regional company turning out good dogs that the FF joints could use as a supplier in each area.
Agree with the mushrooms
Fish and chips, the battered variety of fish
Different flavors and cuts of fries. Dipping sauces for fries. Gravy for fries
Deep fat fried fruit pies, dagnabbit! People were skinnier back in the days when they served up those dessert delights!
Onion rings made from onions, not onion flavored mush
A return to the old, original production techniques the big burger chains once used, and built their empires on.