I agree that you should get a fish YOU want and appreciate, rather than looking for something to wow others in the hobby. Other than being an unusual fish, I understand you are looking for one that is reef safe, easy to care for and suitable for a 57 gallon tank with some smaller fish. I'm assuming you have a cover, as most fish can and will jump, eventually.
As others have mentioned, a wrasse may fit the bill. Radiant wrasses are beautiful, pretty hardy, compared to some others, and I don't see them much. They do need a sand bed.
Mystery wrasses are also one of those that aren't really rare, but I don't think they are super-common. They are active and really "pop" in a tank. Not sure if one will be too aggressive for your firefish.
I don't know how much flow or what types of corals you have - but if flow is moderate and you have corals without aggressive stings, you could consider captive bred seahorses (disease free, acclimated to captive conditions, trained to eat frozen). I buy from seahorsesource.com. They have excellent stock and great service and support. Seahorses have specific requirements for flow, food, tankmates, so be sure you research first.
Have you considered a pair of ORA captive raised mandarins? They are healthy and are used to eating frozen or pellets. I have a male target mandarin from ORA. He is thriving in my 44 pent. Wild caught mandarins often starve to death in smaller tanks because they don't recognize frozen or pellets as food, and quickly decimate any pods.
They have several species. It's not so uncommon to see mandarins, but a vibrant, healthy pair is spectacular, and not necessarily common. In a peaceful tank, they are really noticeable and active.
I would avoid fish that will hide, like assessors, and any that are especially picky eaters or have a bad record of survival in captivity (WC pipefish, moth fish, shrimpfish, orange spot filefish).
I have a marine betta, which would work in your tank. It's not rare, but I don't see a lot of them. It is hardy, not aggressive, and really spectacular looking. They have a reputation for hiding a lot, but mine is out quite a bit more than I expected. Being in a peaceful tank helps a lot.
I hardly ever see mature engineer gobies in reef tanks. If you have a good foundation of rockwork (resting on the glass, not the sand) consider an engineer goby. They are really cool looking when mature, very peaceful and hardy. Interesting burrowing behavior. Great eel "substitute" for a peaceful tank.
Good luck with your search for the perfect fish! I am partial to puffers and filefish for their cool personalities and looks, and to lionfish for their predatory elegance. Not sure either would be a good fit in your tank. If you could find a pretty reef safe small filefish, it would be a great additions. I like these.
Mimic filefish (mimics a puffer - beautiful, small, I suspect reef safe). Nice in pairs, but buy them as a pair.
Bristle tail (aiptasia eating) filefish - small, peaceful, do well in pairs, great personality. Not sure how reef safe they would be.
I had one of these. Very active. Though not big, not sure your 57 would be large enough.