If you do any serious renovation paint it a light color use light colored carpets. Just light colors in general. Maybe get some low voltage halogen lights in the dark bit of the room to lighten it up. Also if you're short on space you could try and fit a captain's bed in there. They're like a bunk bed but with only the top bunk. The bottom bunk consists of a desk and a futon. I don't know if you'd be able to fit it in there though as you said about a sloped ceiling. You could just try putting a normal bed in the lowest part of the room and keeping the tall-space for other stuff. The lowest part, as you said is the darkest part so why not sleep in it?

Just a few ideas taken from my pitifully small room.