If you want to do something simple, a gradient toward light or possibly rays of light from the upper left would fill that space. If you think that's overdone or wouldn't work then maybe some sort of architecture? I like how it's looking though.
I think this is good. The smooth coloring and the shading are well done.. How about a big screen TV up there..and he's watching reruns of the Lucille Ball show
drawn in 1 hour 42 min
drawn in 25 min
drawn in 22 min
drawn in 12 min
drawn in 39 sec