I second the suggestion of irises. The image in my mind is of swaths of various colors of Siberian (and perhaps other types of) iris. And Cattails. I don't know anything about tall-growing grasses, but something in this category might be interesting.

Keep it simple...

Just think what will happen when we finally do get a little shot of sunshine.

Maureen

Two deciduous rhododendrons: rhodora (R. candense) blooms magenta in very early spring followed by blue-green foliage, and swamp azalea (R. viscosum) blooms white to light pink summer and may have nicely colored fall foliage.
One evergreen rhodie, R maximum, is moisture tolerant and blooms about now. There are a bunch of selections for flower color, etc that you may be able to find at specialty mailorder nurseries, that might not but most likely will have the same moisture tolerance.
Labrador tea (Ledum groenlandicum or Rhododendron groenlandicum) has evergreen foliage and small white spring flowers.

High bush blueberry (Vaccinium corymbosum), has flowers, fruit, and red fall foliage
Huckleberry (Gaylussacia frondosa) is similar to blueberry.
Red chokeberry (Aronia arbutifolia) has white spring flowers, red fruit and fall color, and black chokeberry (A. melanocarpa) has similar characteristics with dark berries.
Serviceberry or shadblow (Amelanchier spp.) has early white spring flowers berries the birds love and may have nice fall foliage. Different varieties are from large tree to large shrub sized.
Wintergreen (Gaultheria procumbens) make a nice loose evergreen ground-cover with small white flowers and red fruit.
Summersweet (Clethra alnifolia) has late summer flowers in white to pink, and many selections.
Red osier dogwood, (Cornus sericea or C. stolonifera) has bright red stems.
Northern arrowwood viburnum (V. recognitum) has nice panicle of white flowers, followed by berries if there is more than one variety in the vacinity.
White cedar (Chamaecyparis thyoides) is an evergreen that likes wet feet and you can find selections from large shrub to large tree in size.

Some other perennials include turtlehead (Chelone), white to pink late summer flowers; cardinal flower (Lobelia cardinalis) scarlet late summer flowers, marsh marigold (Caltha palustris) yellow early spring blooms, several kinds of gentian, fringed, closed or bottle are two of them, and many kinds of the fibrous rooted iris, like Japanese, Siberian, blue flag.

There are several ferns, like cinnamon (Osmunda cinnamomea), Royal (O. regalis) and interrupted (O. claytoniana) that are lovely and like wet feet.

Regardless of what you put in, I'd add organic matter to your wet area before or during planting, since that will help keep it evenly moist, and mulch it for the same reason. I'm glad you want to work with what you've got!

Yellow flag iris is a banned plant in most states, proceed with that one with caution. It has ambitions of taking over the world and chokes out natives with ease.

We had it for years (and likely will for many more ;) ), but yanked it and reworked its site a year ago. I have been alert for its emergence and have brutally suppressed it.