It depends on where you want to go. If you want soft corals, Capnella (kenya tree), sarcophyton (toadstool leather), sinularia (finger leather), xenia, or anthelia are all good starters. Many kinds of zoanthids (button polyps), star polyps, and mushrooms will also thrive in your system.
If you want to take a stab at stony coral, then euphyllia (frogspawn, hammer coral), montipora digitata, monti capricornis, or pavona cactus would be good. Different kinds of brains will also work. You will have to keep a closer eye on you Ca and alk levels than you would with soft corals, though.
That's a start. You really have a lot of choices.