Trying the picture again. SORRY!!

Here is a link that might be useful: {{gwi:1254805}}

A blank slate! I love blank slates and am a bit jealous. However I do not recommend filling in the the timbers with soil once you have the additonal timbers in place. If the tree were younger or a new planting, then I would say go ahead. But you have an established tree that has many feeder roots out in the timbered area. By placing additional soil on top you will bury these roots depriving them of the oxygen they need which will either injure or kill the tree. Take a lesson from me - I had an established red bud on a slope. All we did was put in a retaining wall and even up the slope, only covering half of the tree roots with up to six inches of new soil. The tree died within a year. I would suggest taking the timbers up to 3 as you want, adding in plants and then just mulching the area. As for plantings, are you looking for a green area like the hostas or to add color? Perennials, annuals or both? Possible perennials (I am guessing this is dry shade area because of the tree roots): Lady's Mantle, Columbine, Bergenia, Brunnera, a mat forming Campanula to hang over the timbers in spots, Coral Bells, Tiarellas, Epimediums, just to name a few. Annuals: impatients and begonias - and there are lots more.

I hope you post some pictures when you finish the project.

I have hydangeas, fern, azaleas and a few day lillies with my hosta and it's so peacful and pretty. I also have (I think this is the name) Japanese sedge grass flowing over the brick wall in several places along with a few of the things listed above. My bed is LONG - about 80 feet - and is in good dappled shade/morning sun and a bright shade the rest of the day. It works great for me.
I hope yours turns out beautiful.
Sunny