Yes, more ground covers. Walli's 'Mouse Ear Chickweed' is a good one. More like this can be used, especially if they are clumped and grouped. You can find plants like this growing in the wild (some flowering) in all weather zones. They make great transitional plants in between the larger bushes and trees.
As for Hero Trees, I plan on scanning a few pages of my bonsai manual that illustrate some great ideas for picturesque trees and post them here. I'll have the images for viewing as soon as I get the manual back from my granddaughter.