Downstairs offers a bonus bedroom with a full sliding-mirror closet and central A/C and heating. The space can be used for anything from an office, exercise room, extra teenager bedroom, extra storage, or even as a possibly expandable guest quarters/studio unit.
Due to being downstairs and separated from the rest of the home, with very simple modifications, this space could be expanded with a kitchenette and it’s own separate entrance, or just be left for extra storage. Let your imagination or needs choose.