Well Topic Creator (What the world does OP mean? Original Poster?) I have to say you know how to type to get people interested in what you suggest.
Anyhow I can't let myself be drawn in by flavor text. So let me look at it closer
Ghouls and demons that pull your dwarves into the depths, to join their ranks
-Undead I can understand. Though what are demons doing there? I can picture a certain demon doing so but they are kinda limited in both what they can do and how they can be there.
Even the elves advocate burning these places down
-I'd personally would only suspect they would do so if this play is tainted by unnatural forces compared that they worship landmasses. Unless they somehow make it their version of an evil god but even so.
No magma will destroy these dark places
-How is anyone supposed to get Magma all the way out at a swamp? Though the important question to ask is "Why not?".
If you sleep here as an adventurer, you may very well find yourself buried by the psychedelic fungus feasting upon your flesh.
-A knowledge of the landscape should protect you from this. It would be interesting if survival was actually needed in the wild.
Hermits walk the depths of these places, luring nearby travelers to their doom
-Ok ok at this point EVERYTHING in this Swamp is out to kill you. It seems more appropriate now as a description of some sort of Entrance to Damnation then a swamp.
Now build a fortress on it.
-It should rather be impossible since there is nothing to build a foundation upon. Though it makes me wonder if there should be places so dangerous that they allow you to take more people with you when you Embark (or some way to have larger Embark crews such as funding them through fortress mode)
Though I wonder, should this be a Spherical mutation of a Swamp or a entirely different kind of Swamp all together? Something "Beyond Savage" which I guess could be expanded to all locations that represent the danger levels of an area being heightened to beyond earthly limits.
Overall I like it, I am sad that people didn't open up this suggestion as a in depth discussion on how we can construct dangerous swamps.