there are quite a few
trees and plants that grow under or near black walnuts. (dead ones too) around here mulberry works well and gives a good crop of fruit, biomass, coppice
wood and more. if you Google black walnut tolerant plants you will get a good list of websites with plants and trees.
fungal decay is your best bet for "regenerating" the site for other plants. you could use wood chip mulch and
chickens to increase biological activity below the soil. over time new species would thrive there.
putting a path or other physical element(patio,outdoor kitchen,coop, etc) is also a great idea given it flows into the design well.
