The house I grew up in was built in 1923 and 2 of the bedrooms had sloping ceilings. In the front bedroom the previous owner made that wall a closet. (Wish I had pictures to share - but we no longer own the house.) The closet was the full length of the wall - and was so long and the ceiling was low - there was one small door at one end, so my dad put in sliding doors in the middle along with a couple of lights.
A couple years ago I was watching one of those shows that renovated what they called wasted space. They did about the same thing my dad did, but they put in shelving too.