Long term, any trigger would need a bigger tank. 55's simply are not large tanks and triggers are fairly large and active fish. Crowd them, particularly with other fish and the aggression could be intense. In a species only tank, with perhaps one fish, an undulate would be a consideration. I wouldn't plan to keep any other trigger in a 55 for life.
The eel, puffer, maybe a hawkfish or large maroon clown would be cool. Another often overlooked fish are the hamlets. Colorful and a good mix for a smaller aggressive tank.