If you are looking for a setting dynamic to a swamp. I have three suggestions.
Firstly, undead should not be overlooked, and not simply as a mechanic of the fact that undead should be in such a place, but that the living shouldn't.
Second, bandits, rebels, exiles, and criminals are often driven to such places so that the swamp will take care of them.
That last one can be its own plot, lead to the first plot as a group goes undead, or lead to my third suggestion.
Third, Fey are often swamp creatures. Lots of them are aquatic, lots of them are nasty, and they have no specific nutritional requirements to make the place nasty. But the story I would build around them is that they were some sort of mortal or more normal group and that they did something or had something done to them that turned the land horrible around them. Now they live on the edges of the reality of it and are hostile to everything. Particularly mortal interlopers looking for their ruins or the undead they harbor.
Subservient to all of those is that as the new group moves in the wildlife becomes weirder in response. If I were a power or a wizard a swamp would be great since you can fortify your own tower to keep the creatures and swampness out, fill the surrounding land with utterly nasty stuff, and then just port out when you needed to go somewhere.