Here is my two cents:

Jawfish, yellow headed or pearly white - moderately easy but needs a deep sand bed, 5" - 7", pretty shy too.

Mystery Wrasse - easy but way expensive

Forktail Blenny - ok but can be aggresive

Chalk Bass - good choice, easy

Fingered Dragonet - very difficult due to the need of a high population of pods for food. Probably should skip this one

Blue Flavivertex Pseudochromis - good choice, easy to keep.
Randles Goby (I love mine in the nano, so I need another) - good choice

...and maybe a blenny - I would recommend a lawnmower. Not very pretty but they have great personality and work hard as a algae eaters.