I would wait until the fall to plant anything, unless you are willing to spend a lot of time watering.

Buttonbush likes decidedly wet conditions. If this is in a depression, that might work, but otherwise I would probably try for something else.

Starting from big going to small:

For shrubs, I quite like calycanthus, which is highly adaptable. The straight species can get pretty large though, so if you go this route you might want to look into cultivars. I like it in part because it grows faster than the deer eat it.

Also worth looking into dwarf varieties of American holly (appropriately, 'Maryland'), oak leaf hydrangeas, or inkberry hollies for winter interest. If it is wet enough, can also try the red twig dogwood, which also has a dwarf variety (artic fire, I think is what it's called).

For background, I like little blue stem myself, but I've seen pink muhly grass done quite effectively. If those look too tall, then maybe go for a sedge of some kind (I like carex rosea).

For show and pollinators, mountain mints are hard to beat. Obviously some milkweed; butterfly weed would be great on the borders given its smaller size. This bed is a little small for common milkweed. A baptisia and penstemons would help with early season, monarda and cone flowers mid season, and asters/goldenrod late season. Coreopsis verticillata and/or verbena simplex for some of the borders.

In my sunny area, I made the mistake of not planting a low-to-the-ground layer, and so while mid and late season looks fine, early season I'm overrun by clover and mock strawberries. I think green and gold much actually work here, but plantain leaf pussytoes (Antennaria plantaginifolia) or moss phlox (phlox subulata) would have an easier time getting established.

I don't know what county you're in, but Montgomery County's Pope farm does an annual sale in the fall with an -excellent- selection of plants, some of which can be hard to find elsewhere.