Build a shadow box frame with glass on top and put some art objects in it. Or just paint it to match the walls and ignore it. If you have a cat, you can add a couple of cat perches. Colorful artwork on the wall will distract from the sloped area. Also, with such a beautiful window, no one will notice the slanted area much at all.

Exactly what njmomma said. Remove the light and drywall the area.

What is under the slanted area?

Yes, what is in the wooden slanted box? Easiest fix would be to remove the light and paint the area the same color as the walls.

Remove light drywall area

You could put in a built in bookshelf or display case of some sorts.

Someone was obviously considering installation of a slide. I think you should complete their unfinished project - what fun!

Private Residence - Disney's Golden Oak · More Info

Gibbes street · More Info

Sidebottom Residence · More Info

decoenthusiaste - those are a riot!

As was mentioned - a look like this with shelves to match the line of stairs - then possibly bookcases across the wall to the door - doesn't have to look 'too much'. Guess what? I still read books, paper books.

Psinta Dark Brown Modern Shelving Unit · More Info

Variations - one has cabinets at bottom.

Staircase library · More Info

Reiko Feng Shui Interior Design - Loft renovation · More Info

The slide is cute --but that is an odd area, it must hide some vent or ductwork?? Oh well, if possible, I would install shelves horizontal from the room edge, going to the back about 2 or 3 feet apart stacked so that they stop about 3 feet from the ceiling. Than, I would get baskets or bins, for seasonal decoration storage, or luggage storage. Depending on what you decide to stash on the shelves, and what containers you use if any, you can have an access door installed that opens into the room, remove the banister, and put up a wall there. If you want you could make the door a blackboard paint feature, or display artwork on it or a corkboard for casual photo display, etc, if that is something your family would participate in, or just stain the door or paint it, as well as the new wall on the stairwell, and do change or remove the pendant light....... Now that I have thought about it again, perhaps it was left open to allow for wider objects to be brought into the room from above, if the stair rail was removable? Nice big room and a great window to enjoy and let light in. What will you decide? Enjoy the challenge! :D