You could add organizational units to the cupboard, which helps with storage. Use bins under the bed, a 4-5 hook rack over the back of the door to hang things, a trunk at the bottom of the bed for storage, etc. Take advantage of any extra floor or wall space. Add shelving on wall and add baskets for example, or an armoire or chest. Where the ceiling dips, add a low console along the wall. If you type in attic bedroom storage, you may get some inspiration.

Do you have floor space to actually widen the closet itself?