At the risk of incurring the wrath of the tang police, a juvenile Kole or Tomini tank wouldn't be beyond the pale for a 56 gallon tank. If you chose to do this, I would recommend you center your rockwork so the fish would have 360 degrees of freedom to swim (one large circle, in other words...) and build the rockwork UP to compensate. Keep in mind that either fish would eventually outgrow the tank, so make arrangements with another fishkeeper or LFS to swap out....possibly in as little as 18 months from initial purchase! If you have a solid reputation with a LFS, you might convince them to "trade in" your subadult tang for another juvenile, thus continuing the cycle.
I would only suggest trying this AFTER you have some experience under your belt with your tank. Give the new tank plenty of time to mature, and give yourself time to really get used to the little quirks of this particular tank as well. Also, I'm serious about the species.....I don't think there are ANY other tangs that would be acceptable. Consider the Tomini....a very beautiful small tang, and you could call it a "gift to yourself" for a job well done when the tank is a year old.
Hey Spanko, you remember the tang police GIF with the car? You know, I remember someone adding a GIRL in the car as well...nudge nudge, wink wink, say no more.....