Most of the shell- dwellers from Tanganyika would fit that tank as well. After that.... not much else cichlid- wise. Here's a deciding factor: plants or no? If yes, look into the Kribs ( get a bit large for my taste in a planted 20 ), Rams ( you can often get by with larger inverts such as Amano shrimp with them ), or Apistos ( sp. cacatuoides ). If no.. look into shellies. Hard water or soft? If hard the shellies or kribs will do fine. If soft look into Rams.
You could upgrade to a 29 and some more options open up for you..... (convicts, labs? )