Heniochus are beautiful fish and easy to keep. They grow too large for a 55 though.
As mentioned, be sure to research the needs of any butterfly that you consider. If you keep your tank lightly stocked then you can probably get one down the road. Some possibilities are: Burgess, Klein's, Bluestriped, and possibly copperband. Copperbands are hit and miss though. Some do well and others perish within six months.