We haev a storeroom. It doesn't slope, but it has an area about 3 feet high & 2 feet deep, and the rest is pretty normal sized.
You can build cubbies into one of the long walls, like bookshelves but little sqares you remember from elementary school. You can use the cubbies to store folded clothes, books, toys, sheets, towels, etc. I would maybe make 1 row for each person or whatever. On the other wall we have rubbermaid type containers for longer term storage (baby clothes, winter coats, etc). We also have a baby dresser in there stuffed w/ baby stuff since we don't have room for the dresser in our apartment.
We use the low area for stuff that is akward to store, like the baby swing. You could also use it for toy rotations or what not.
What types of things do you store? Is it long term or daily use? Our storeroom is up 2 flights, so it's not where I would keep my t-shirts, but it's not exacty inaccesable.