Personally, I would make it a lagoon tank. I would fill it with 6" of sand. Then I would put a few mangrove trees in it so they can grow big and have their root systems branch inside of the tank. Then I would have a couple of pearly jawfish, feather dusters, couple of brain corals, maybe an open brain. I would have a couple of urchins, a queen conch, snails, scarlett hermits. sea cucumbers, nudibranchs and feather dusters. Feather dusters everywhere! For the other fish, I would have a school of chromis I wouldn't have but maybe one large powerhead on the tank. No filtration besides the deep sandbed and the mangroves.
Something else that would be cool in this setup is to put a nice couple of light weight rocks in there to build up one side of the lagoon out of water and have a mangrove stick out between the rocks. I would then put in some land hermit crabs so they could go down and get wet and then come back up and bask in the light and have a place to stay. Not sure if they would be tooo agressive though, never done it before.
I personally like the more unique ideas.