I was originally going to do a salt tank to upgrade my other reef tank but nixed the idea mainly because of water changes and the mixing tanks I would need. I don't have much experience with brackish tanks but I would assume its similar.I would try to find 3-4 Ceylon puffers (tetraodon fluviatilis) and make the tank brackish. They would look awesome, using the height of the tank for sure. They are very active swimmers.
I have a SA/CA tank now, but what about some more aggressive fish? Maybe a piranha tank, not sure how many it would be able to support and I would love to do pbass.