650gallons?! Am I hearing you right?! That's huge! And exciting. And I'm a teeny tiny bit jealous.
Anyway, have you kept triggerfish/puffers/eels previously?Those aren't really a challenge but fun, and intriguing to feed. Any type of butterfly fish (ornate, threadfin, raccoon, copperband etc) could be considered difficult unless you entice them to eat.
Filefish are also interesting additions :P as well as boxfish, large angelfish and harlequin tuskfish. Those are just ones that I like anyway. If you're looking for the mother of all challenges, then a moorish idol would be your answer.
I don't really recommend that though, because moorish idol mortality rates in captivity are depressingly high. Anyway, that's just my opinion