I am having a hard idea coming up with ideas of what to plant and how to make this look full and luscious. I am planning on starting part of the area over. There are some plants in there that do not belong.
Zone 5a Colorado, part shade, clay loam, pH neutral. Some things that are in there are:
Northern sea oats, coral bells, tri-color beech, limelight hydrangeas, hostas, and periwinkle.

What about ferns?

here new member. This is a safe website to help you get an idea of what you might want to plant.

https://www.gardeningknowhow.c...

I will definitely add some ferns. Thank you for the link as well! I think my hang up is how to layer it in an appealing way.

Your tree is improperly staked. Remove the stakes. If it can't stand on its own in that location, you bought a weak tree. If you must stake it, use stakes that are easily removed, use three stakes, not two, go about a foot lower than it currently is, and use something soft and stretchy about the truck like t-shirt fabric or old panty hose. It is difficult to see what type of tree it is, but there is a possibility it was also improperly planted, as most trees appreciate good drainage it it would have appreciated being planted on a small rise.

Think of sweeping lines on a page. This is appealing when used with plant material. It makes a sweep of color in the landscape. If you lay the plants out with the tallest at the back and shortest near the front you will get maximum visual effect. The different colors and textures draw the eye along.

I would choose 3 colors that complement each other so no matter what is blooming the plants will be cohesive.

Ive some trees staked for years not because the tree is weak, but because the soil it was in was too soft. There doesn't appear to be anything wrong about the way this tree is planted. The important part is to make sure the root ball is spread out so that roots are not entertwined around the truck circling it. And we can't even see that from a pic.

Have you looked at some of the alpine plants . Colordao is a high desert plateau.

https://www.gardenersworld.com...

Or alpine and rocky plants. Create a beautiful Rock garden.
https://www.daviddomoney.com/r...

Spiderwort (Tradescantia )
Hellebores
Brunnera macrophylla 'Jack Frost'
Monkshood (Aconitum napellus) ...
Columbine (Aquilegia )colorado state flower.
Hostas in many colors shapes and sizes. See hostas catalogs.
All the above do well in shade and I have had or currently have them in my garden.

Add a few under story trees and shrubs.
https://www.thespruce.com/tree...

If you plant all perennials and annuals as one layer that are great for shade, then the space for understory small tress and shrubs would make another layer of plants , this would be a great way to fill in the space under the taller trees.tallest in back and so on. Remember very tall trees that exist in your garden will be thirsty and rob grasses and plants near them the new plantings will need a good watering system in summer. Especially until the new plants are established.

Happy Gardening.

you could add Little Quickfire hydrangea, they do look altogether different than the limelights. You may consider some ever green shrubs for areas that don't get too much wind.