@Stephen R I'm not able to do quotes for some reason, I'd like to contribute to your question, but 1st let's get an idea of what you like. As all fish are really predators when you get down to it, the term is thrown around a bit loosely for my liking. As
@Nokiaec11 eluded to, a predator tank and an aggressive tank is different yet mixed and interchangeable.
I keep true predator tanks, which mean I feed live foods. Have you read my recent post in the Predator section, you may find this interesting.
https://www.reef2reef.com/threads/an-arguement-for-feeding-live-foods.582822/#post-5945226
Alot of the predators like the dwarf and medium lions, scorpionfish. waspfish, anglers and such would benefit from live foods and some maybe impossible to keep on dead food foods long term. All of these type of predators can live well in a 120g, mixing them is somewhat tricky but doable.
Now if you are leaning to the aggressive attitude fish, any of the triggers would get too large and aggressive for a 120g. Some of the smaller eels would work. Hawkfish are overlooked and quite deadly, but because of their relatively smaller size would limit you to some other choices.
First off, let's get a direction to what you like.