Po-Boy in it's purest form....French Bread, fried seafood of choice, shrimp, crawfish, oyster, catfish and soft shell crab are popular ones. "Dressed" = Lettuce Tomato, & Mayo, often times pickle. That's pretty much it. Next on the list would be Roast beef po-boy,(sloppy with juice) Meatball has a following as does the French Fry with gravy.
After that you may see/hear of any number of combinations of meats, condiments etc in an attempt to re-invent the wheel. My opinion...stick to the basics!
PS. I do sneak in a tiny dab of Creole Mustard on occassion.