It can include characters from the National Treasure movies or not. If in the National Treasure universe, Harry could maybe be a bodyguard or security for the treasure hunt. Maybe even a tour guide. Fleur could be a historian on long lost artifacts. If its set in the HP universe, it could take place after the war. To repay the goblins for the damage to gringotts, they task him with finding various artifacts. Fleur, having worked there previously, is assigned to help him. It's not really a crossover with National Treasure but you could take some ideas from it.