As for regional items on fast food menus, I’m sure it’s come up in other threads but for the past several years MacD’s has done a lobster roll in the Summer in New England. Never tried it, but the reviews say that for the price it’s not bad. D’Angelo’s, our regional sub chain does one as well.
In the 60s, there was a Rhode Island mini-chain called Kelly’s (NOT Kelly’s Roast Beef) that did R.I. clam cakes. Not a cake, and barely a clam, but a ball of fried dough with a few pieces of white rubbery "clam" inside. Kids went nuts over them.