I've never tried basing this kind of thing before, but I wonder if this might help. There is a big difference between a swamp (wetland with lots of tree's) with other types of wetland - your modelling could reflect this?
Eg: a Marsh is (if I remember correctly) a wetland that's very prone to constant flooding. So, model this up as more water, slightly murky and muddy - have the models literally drag themselves from the mud - to model this up would mean lots of greenstuff on the base with lots of brown washes?;
whereas a Fen is an area more willowy type tree's, often leading from a lake to dry land, meaning more normal flock with areas of standing water and grey to white tree branchesor;
there is a Bog with more acidic content - lots of yellows on the base will work to make it look right with lots of brown water - also lots of dead plant material (fallen tree's/branches etc). Interestingly, there's lots of berries apparently in fen areas, and some spooky sounding carniverous plants such as sundew and butterwort. remind me never to get lost in a bog!
I suspect Hell Fen, from it's description, is more of an acidic bog.
Some other ideas you might want to use - build some glowing marsh lights, a bit like balls of will-o-the-whisp's? the other thing is... well, the undead used to have units of mummies from Araby. There are however, preserved bodies in peat bogs that are as well preserved as any mummy - have a blackened and wizened wight king on foot modelled up to represent one of these bog mummies? A wikipedia search for "Tollund Man" might give you an idea of what I mean.
Best of luck!
Edit: just forgot - you might want to also think about incorporating luminescent fungus asa light source on yoru bases - wrap some greenstuff round a model branh, dimple it to make bubbles of fungus, and paint in bright luminous greens that cast an eerie light on the model - this is called "Foxfire" in some places I think?