TARDIS Exterior: The Sandcastle TARDIS
Why I went with this design: I thought it would be cool to see a TARDIS materialize on a beach as a sandcastle, with the door getting down like a drawbridge. Also, I imagined it with a bucket on the top and one of the tower collapsed to make it look like it's still in construction.
Images:

When I drew this, I gave the bucket a round shape to match the Bucket item, but if it gets modelled in 3D, the bucket would be more square shaped to not have a model too detailed.