You have nearly unlimited possibilities for a 55 or 75 gallon
only thing I can think of right now that'd off limits are common pleco's, koi, and I believe most sharks. Someone else had started a thread about a 55/75 upgrade a member told me to look up an albino longfin bristlenose pleco.. And it's absolutely gorgeous! Personally I would do freshwater, but I don't have the time to keep a saltwater aquarium. I think going with natural decor would look awesome, also having live plants would be good ( if you even want to go that route).