Keep in mind that you already have 3 large growing fish, two of which are quite active swimmers so I would not suggest more than one addition depending on what you choose. Large angels, smaller eel species, marine betta, scorpion fish or possibley one of the larger growing puffers from the Canthigaster
As I am sure you may already be aware, the types of fish you seem to enjoy are large growing and often times messy eaters which can make it a challenge to get the right mix. At the next (affordable) opportunity, you should consider upgrading the tank size to a 150-225 gal
tank. It would allow for much more variety as well as being easier to maintain overall.