im thinking that the girl could possibly be the daughter of Andrew Ryan, looking out into the place that made home to her fathers dream. The big daddy doll replaces the teddy bear that a typical girl would have. The doll is something the girl is attatched to, therefore showing her attatchment to rapture. Building a sandcastle is a creation of the imagination, and since it is clear she is doing it with her mind, we know that it is a product of her flowing imagination as she stares out into sea. This is her dream, to revive her fathers dream and try to create the perfect utopian way of life. The butterfly emerging from a barnacle,(a barnacle being a product of something stagnant and unlively underwater) shows that Rapture will be rebuilt and the "dream" brought to life. catepillars usually emerge from a cacoon after a period of hybernation and change into lively, colorful, butterflies. This is exactly what rapture is going to do after its hybernation, turn into a colorful and lively utopia. The fact that the sandcastles were built out of dry land signifies that rapture will be reconstructed on land. who knows what will happen with the new leader and her "sea of dreams".