Andorians - I'd love to have the 4 sexes made canon.
Bolian - You see them everywhere, but we don't know that much about them.
Ocampa - Post-Caretaker would be pretty interesting to look into.
Sheliak - An interesting species we unfortunately never got to see again.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)