I'd recommend summersweet clethra. It comes in white or pink, grows to about 5 or 6 feet and has fragrant flowers from about mid July to mid August. This plant has a very clean appearance even without pruning

Some cool options, although you might have to hunt around a bit: There's an American Holly called 'Susan' which is naturally somewhat small, growing to about 15 ft with a dense conical habit. You'll need a male to get berries, of course. Hardiest male I know is 'Jersey Knight', which gets taller but is easy to keep pruned. There's a very beautiful red cedar (which is actually a juniper, Juniperus virginiana) called 'Lebrotni'/'Lebretonii' (the name appears to have morphed depending on who you read) that's an intense silvery blue-green and only gets about 15 feet tall. There are lots of good native viburnums. Some types that like dryer conditions and part sun such as your site are V. prunifolium (Blackhaw Viburnum), V. rufidulum (Rusty Blackhaw -- if you're Z6 it should do fine for you) and V. dentatum 'Autumn Jazz' or 'Chicago Lustre'.

Hello, Well, unfortunately, we're talking about only 24 feet of open space along the property line. The main reason for planting is to hide my neighbor's backyard because it's looking like he's planning to spend a lot of time outside. So, it's his back door, which is located right in the middle of his house, probably right where the open space begins. I am planning to have the existing arborviti cut back so they're all around six feet. Does that help in any transition? Can I put in a flowering something where his back door and then continue with a different series of something hedge or trees? Thank you.

Ligustrum japonicum 'Texanum' is a lower and slower growing cultivar, often chosen because they don't get as tall as the species. It is more for locations where owners do not want it to exceed 7-8'. Yes some will get a bit taller with time. You are asking specific size and growing questions, but don't seem to understand that there is a variety of growth conditions that are impossible to calculate. Even individual plants, growing side-by-side with the same growing conditions, can grow at different rates. Besides the water, soil, nutrients, individual plant vigor, pruning, root systems etc. over time, no less, we don't have any way to know the quality of the nursery stock you ultimately will purchase. We can give you general guidelines, but not exact growing rates for hypothetical plants and conditions for their lifespan. No one can.