You could house a couple of dwarf lions and make the rest of the inhabitants a bit more out of the ordinary.
As of other fish, choices are extremely limited and in a 55 gallon, limited all the more, but things like sea urchins, snails, larger hermit crabs, cucumbers, starfish and even simple coral can make an interesting tank...provided you don't decide on a puffer or a trigger. Neither really being suitable for a 55 anyway except perhaps for needle nose puffers. If you do decide on a puffer, careful with introduction. they should pretty much go in at the same time...both juvenile to have the better chance of avoiding unwanted predation or aggression. The puffer should always be noticeably bigger than the lions mouth.
Dwarfs especially do much better in natural reef settings...live rock and live sand. Deep sand beds can be beneficial for nitrate control provided there are enough organisms living in the sand to maintain the bed. Good water flow and oxygenation is very important for bulky fish. They utilize much more oxygen than the average fish.
Try to get a specimen that eats thawed frozen foods. You can always treat them to live ghost shrimp from time to time, but stay away from live fish as feeders. When these lions get sick...they go down quickly and are very hard to medicate without making them more sick. Frozen squid, prawn, silversides, and beef heart are eagerly taken once trained. Keep diet varied.
Use live rock to cycle the tank, then use a few damsels (no more than 5) to settle the cycle before adding the lions. Lions should go into a QT for two weeks...bare tank with a non porous object for hiding. Use a simple sponge filter...rince daily. Use a half dose of a mild antiseptic. Avoid copper treatments.
I would personally do a freshwater dip for three minutes (maxiumum time is five minutes)to ward off any external parasites. Some people are touchy about the practice, but I find it works well and worth the stress, especially with fish that are copper sensitive such as lions. If you decide to do a FW dip, make sure the pH and temperature matches between the hospital tank and the freshwater dip to lessen shock. Run an air hose in the dip.
Take your time and cycle. Read and ask questions...even if you think it's silly. These fish may be tough in looks, but touchy to water quality.