it depends on what type of fish you're looking for...
damsels are kind of just a** holes... i don't remember the swearing rules so hopefully i don't get written up for that... but they just nip at everything kind of like freshwater barbs... they don't really eat smaller fish..and they don't get huge...but they just kind of suck.
chromis are omnivores....but they are REALLY peaceful and this is normally what people tend or should start up with ...they are hardy and more peaceful than most reef safe fish...if you did plan on adding corals ever...
a 40 is really too small for any type of angel/trigger combo/setup.
frogs may be ugly, but they are ambush preds and can live in small spaces...
a snowflake eel needs a little bit bigger than a 40 gallon, so if you can rehome him in the future to a 75 or so... that wouldn't be a bad starter pred...
groupers get huge so thats a no..
hawkfish and dwarf hawks might not be a bad choice...not super aggressive, but they are carnivores.
lions need way bigger tanks
dwarf lions will also need more territory so i would stray away...
mostly for predator tanks you need at the very bare minimum a 55 gallon and even that is starting to push it ... i would recommend a 75-90 gallon tank...