Have you thought about making the window wall an accent color? That would attract your eye to that wall making it a focal feature and complimenting the architecture. Rather then how it looks now. Right now it all appears bottom heavy and making the ceiling line narrow. You can't bring the ceiling color down on the top half of slanted wall because that would be too much ceiling and not enough wall height. By having that wall an accent color it will visually wide the room space and keep other walls a lighter complimentary color to the accent wall or a lighter shade of the accent color but in the same family. That will lift, brighten and widen the room look. Give more breathing room and accent the wall look rather than pull it down making it appear heavy. Good Luck it would be nice if you post photos of the after look once you've made your decision and room is complete!!!