No rules - up to your imagination! At Stapely Water Gardens in Cheshire they once had a large waterfall built from toilet bowls! Not a great artistic success, but definitely different. First decision is are you going to fill it with water or use it as a big planter? If its going to rest on the ground rather than sink into it you may like to think about some low shrubs to surround it and hide the sides?

As a pool you'll have to fill the plughole, and as a planter you'll need to make extra holes for drainage.