be very, very careful adding soil around trees. The root flare should always be exposed, as that ensures that the roots can do their job and you're not exposing the trunk to rot and disease. It's hard to tell from the pic but unless you have an erosion problem, I wouldn't add soil.

In terms of plants and competition, you have it reversed. Anything you plant will be shallow rooted to start, so they'll be competing against the existing trees (with extensive root systems) for resources. Keep those watering needs in mind!

It would be irresponsible to suggest plants without knowing where you're located (location and, ideally, USDA zone)

I agree with Rev Gardens - we need to know your USDA zone.

I am in Peterborough, Ontario, Canada and it is zone 5a.

Rather than plants perhaps a great stone bench and interesting bird bath with a Sheppard’s hook for a hummingbird feeder. Plants under legacy trees are tough to get growing so I would try alternative schemes. Trees absorb a lot of water and nutrients.

You are clearly living in a natural forested area. Native wild flowers or ground cover that like shade would be best. Don't add soil and don't bring in anything that might have worms in the soil. All worms in Canada and N. USA that are found in previously glaciated areas are invasive. They are killing off native forests by decomposing the leaf litter too fast thereby destroying all the micro organisms that the native forest relied on. Go native.