Since no one else is answering, I will give it a shot. Echinacea comes to mind, right off. If the daylilies are pinkish, wouldn't that look nice together? I saw echinacea in Kansas so I know they must be able to handle humidity. Then you could add cleome, maybe? Maybe something along the lines of geranium 'Biokovo'. It is evergreen, burns some for me in full sun, but maybe not in zone 7? I do not know about humidity for that one. It is a ground cover kind of plant. Since my aster is mildewy, I assume that would not work. What about the veronica called 'Red Fox'? I am just guessing at something with the pinkish color and a different shape on that one. For spring, you could put in peonies. Eupatorium would give you some height.
What other colors do you like? Balloon flower likes full sun and blooms all summer for me. I don't like the yellow of rudbeckia but they would be happy in the same condidions.

I love irises. They do well and the lily foliage will cover them up when they are done blooming. I also have lantana that grows through my lily foliage. It is pretty sprawly, so if you want something tidy, it might not work for you. It is perennial here in Oklahoma zone 7. What about daisies? Dianthus would work well there, too. Sage might look nice, too. Especially something dark like "May Night". What about some mini-roses? Are you going to plant some Spring bulbs? Daffs, tulips, and hyacinths would look pretty there.

Hope this helps give you some ideas. You don't mention colors you like, so that makes it a little harder. You obviously like pinks, but do you want contrasting colors or complementary? Do you want neat and tidy or more relaxed and flowing? I'd also look through some gardening mags to get some ideas of color and form.

HW

How about some catmint for some soft blue color in the bed? Nepeta Walker's Low is a nice variety but not small or low, so it would need to be planted behind the DLs. It can be cut back hard several times for rebloom in your zone. Catmint has a mounding effect with upright somewhat sprawly blooms which contrasts well with daylilies. It will grow in dry soil around your shrubs/trees, but tolerates my wet clay.

Catmint also looks nice with the salvias that Hedgwytch mentioned, especially the dark blue ones. The same is true of the Balloon flowers that Debbieca suggested.

Another idea would be to add a few hardy lilies, Asiatics for before the DLs bloom and orientals for after the Dls bloom. Remember though that the stalks need to remain after bloomtime. Too many can be messy looking in the front of the house, although the big showy blooms make it worth it. Try planting 3 or 5 of the same variety of each type in a color which coordinates with the bed color and planting them as one (or two clumps - not strung out in a row) to make them look neat and organized.

Sedums or small grasses or sedges would also provide some longlasting color and their forms work very well together. There is a beautiful contrast between the somewhat stiff sedum foliage and the flowing moving foliage of grasses. They both also work well with the strappy but mounding green foliage of the DLs.

The sedums come in all sizes (from groundcovers to 3 footers), shapes and colors, so there is one for almost any color scheme, if you can provides the somewhat dryish conditions it needs (although again it will live in the drier spots in my heavy wet clay).

Carex (sedges) like more shade and a bit on the moist side, but are very hardy and troublefree. And again there are many sizes and colors from browns, to greens, variegated greens, & glowing yellows & golds. Great as fillers & edgers and stay nice looking for an extremely long time. Some like Ice Dancer (green & white) are nearly evergreen in zone 5. Evergold is a low but wide one with gorgeous variegated fronds that have a glowing effect in the garden.

Some of the small pennisetums (Hamelyn or Little Bunny) or Red Baron grass (more upright but nice red fall color)would work, as well as some of the dwarf miscanthus (Little Kitten) or feather reed grasses (Overdam). These like lots of sun, but the miscanthus prefer more moisture than the pennisetums.

Asters would would great for you. There is purple dome which is fine in humidity and is low growing, and I Wonder of Staffa is a beautiful blue. Chrysanthemums is another nice late season choice. There are many that do well here and many that are pink. Mine look like daisies and they would be beautiful as your serviceberry leaves change color.
With all the suggestions listed here, you could have a very long, extended gardening season in your garden.

GGG

Thank all of you for suggestions, you have given me lots of ideas. I have almost filled the area with daylilies. I ran out of space for daylilies after I divided them. I gave some away, but still had many. I have visions of catmint, daisies, sedums, grasses, spring bulbs etc. It was a help.
Thanks
Martha