How about some staghorn sumac? Year round interest - tropicalesque in summer, vivid color in fall, seed heads and interesting branches in winter! Will sucker (spread) on its own. To keep it from your grass you can just mow the suckers in it. Low growing Junipers are nice too. There are lots of them, from sea foam greens to blues with bronze in winter. Can you terrace it at all? How about just single rows of flagstone or brick. It will look like someone owns it then, especially if you can use the same materials on the rest of your property. Can you post a pic of the berm and the adjoining property? Try the landscape forum too.

Did a similar thing in our first house. Lucky for us I was working at a floorcovering wholesaler at the time so got a great deal on some commercial carpet with a pattern I loved. Got it cut into on chunk about 12 x14 and another about 5x9. Had the edge bound and a non-stick coating applied to the back of the smaller piece. The smaller one when at the stair landing (reason for non-stick) and we put a nice carpet pad under the larger piece and that went in the living room with about 2- 3' to spare around the perimeter of three sides (fourth was open to entry, stairs and kitchen.

The problem wouldn't be the height, but whether the structural members (e.g. joists) were designed to support a livable build-out. Looking at the plans, it appears as though this is the case, but I'd check before getting too far in the process.

You did not like what I had to say last time. Hopefully you will like this better. You don't seem to be wanting a patio or courtyard sitting space and have said you don't want to block the kitchen windows. Think of your rectangle as a picture frame instead. Now let's draw something inside the frame. Something Zen, calm and simple would be nice. Beth H. had some good examples. Find a piece of art, a sculpture, some large garden tchotchke, a fun piece of junk, something you would enjoy to be your focal point in the picture. Plant a drift of native Florida Coontie, Zamia integrifolia in a groundcover of Mondo grass, Ophiopogon japonicus. Add in a big sack of Rain Lily, Zephyranthes bulbs. The big pale pink ones are my favorite. You now have a simple, no maintenance, calming front entrance that will surprise you with small pink tulip like flowers on and off through the year.