For a building such as an amusement arcade, aquarium, theme park facility or electronics store, turning the entrance way into a shark's mouth will confer an exciting, dynamic feeling to the structure. The head can be contiguous with the sheet metal cladding of a building so that the shark seems to be emerging out of the building itself, rather than being bolted on the outside. The head can then be fashioned from one piece of sheet metal curved into the shape of the sharks head with a jagged row of teeth on each side of the door.
If the building is largely smooth sheet metal surfaces on the outside, the entrance would work well as the center of ring of ripples bent into the cladding. Imagine a stone thrown into a lake. The entrance way would be the point where the stone enters the water and concentric rings of ripples would spread out from it, diminishing in size as they get further away. This would give a person entering the building the feeling of penetrating something deep -- yet live and dynamic.
An organic feel could be given to a building by having the cladding continue down from the sides of the building and out over the steps that lead up to it. The cladding would extend unevenly from the bottom edges of the building and meld to the shapes of the steps, like wax melting from a candle. The surround to the doors could appear quite conventional, except warped and molded to further give the impression of melting.
For a more energetic and less minimalist approach, the entrance way could be ringed in interlocking shapes of birds in flight and loose feathers rendered in copper or iron, and allowed to oxidize to create characteristic colors. These shapes can merge into the cladding of the building so that it seems to erupt out around the entrance way. The forms could also continue out from around the door to create banisters to entry stairs, or even a tunnel, drawing people into the feeling of the building.