I like the lagoon style tank. That would work nicely, especially with some mangroves to provide extra filtration, but it's gonna be a little more work setting it up.
Also, A goby-shrimp symbiosis tank?
Or, as I had, a species tank for a cano toadfish? Really cool fish, and also easy to get because they are really hard to sell for most LFS's because they don't work well in many community tanks.