A 55 is tall enough you could do Angelfish, large tetras (Like Von Rios, Diamonds, Congos, or Emperors), a shoal of Cories, couple of bristlenosed plecos, maybe a banjo cat for snail control.
Do a large sand/small gravel bottom over a layer of garden peat. Either that or use Ecocomplete.
Hardscape with driftwood and black slate. Maybe a couple of small flowerpots half buried in the substrate and covered in java moss. Carefully select and place a couple of swords (maybe 2 big ones and 3 to 5 smaller ones), java moss on any cut ends of driftwood, and maybe down in the crotches of it too. Pick some long tall, straight plants and plant one or both of the back corners for the angels to hang out in a really natural way - vals are great for this, as are some of the narrow leafed swords. Then pick some stem plants with contrasting leaf shapes. Star Grass, Ludwegia, Eloodea densa, cambomba, and hygrophilia coymbosa or polysperma are great for that, and Brazilian Pennywort or other Hydrocotyl species are a great contrast with their round leave.. Add a contrast plant, like a dwarf lily or a banana plant or a bunch of red temple. If you have the light for it (150+ watts of good full spectrum light) and CO2 or feel like dosing Excel, you might want to try making a lawn with Microsword, Dwarf hairgrass, Helzine, or Glossostigma.
Alternately, a loach tank would be a lot of fun. Put in a shoal of 7 or so Yoyos, Angelicus, or Striped Loaches. Get 3 SAEs if you can. Round out with some interesting dithers - Cherry barbs, Black Ruby Barbs, Pentazonas, Red Eyed Tetras, Praecox Rainbows - 6 to 12 of something relatively peaceful, but active and tolerant of active fish.
Use a sand bottom, again, and plant with Wisteria with a couple of dwarf lilys, and make a tree or two by tying java fern to driftwood. Lots of caves (red shale, granite, petrified wood).
At this point, if someone gave me another 55, I might set it up with skunk loaches and American Flag Fish. I'd probably go nearly Mbuna with the rockwork, and keep the plants confined to the ends and a few epiphytes on the rocks, with lots of beach in the middle. Maybe plant some of it with hairgrass, maybe not.
You could go African - pair of African Butterfly fish, Couple of Leopard Bushfish, shoal of Congo Tetras. I'd put some swordtails in to keep the predators in fry. Lots of branchy driftwood at one end (Manzanita would be perfect), An onion plant at the other in the back corner, and let the leaves grow out over the surface a bit. Some anubias here and there, and some assorted crypts in the foreground, stems in the back.
