We too had thoughts of such a wonderful lower level patio when we bought this
house more than a decade ago.
There was some large gravel under the deck and we added small gravel to fill in
and make the walking easier.
Since there was much to do to landscape the yard, under the deck became the
potting/nursery area. The deck provided shade during the hotter hours of the
day for both me and the young plants I was propapating. Since the deck faced
east, the plants got good early sun when placed near the outer edge of the
lower area but didn't get the harsh afternoon and west sun (Zone 7-8).
Be sure the slope is away from the house to prevent water issues.