Usually the salt monster fans go after sharks and rays, but with 100 gallon you probably won't have that type of space. I would look into some of the eel species that can be housed with a reef, as mentioned above, as they get big yet don't need tons of swimming room. Also, some of the puffers can get larger, dogface puffers are terrific fish...they remind me of a salty cichlid, meaning they'll notice you and show some signs of intelligence. Scorpion fish, Anglers are also great fish for salt tanks, but they aren't very active...some anglers can hit a foot or more. Heck even clams can hit 20" in the mantle if done correctly. Your best bet though starting out is probably to stick with a FOWLR setup, meaning Fish Only, With Live Rock...it's nature's filter media. Even though they can be tricky, saltwater angelfish can be awesome monster fish for a good FOWLR system...Emperor Angels are truly amazing at 15". You can also explore the grouper class, with the panther grouper being a popular fish and getting quite large at 20", but you probably don't have the room for it in a 100 gallon. Lionfish are interesting and predatory and some can get large...most of the appeal with saltwater is to forge a 'micro-eco-system' and most of the color comes from the reef setups as well, with corals, sponges, inverts, etc. making up tiny cities. I'm fascinated by a friend's tank that has 20 gallons and a pistol shrimp and goby living together...Good luck! Hope I was of some help! PM if you want to chat more!