There are plenty of fish that would work for a 40g. Liopropoma basslets, chalk bass, grammas, firefish, dartfish, flame and longnose hawks, and many types of blennies and gobies.
You need a canary wrasse (also called yellow coris wrasse but terrible name because its not a coris at all lol) but youll need some sand or a container of sand if you do I love my little guy! And I adore my midas blenny too. Not sure how it would work with more fish-those are the only 2 I have in there.