My suggestion is to figure out what tasks/activities you would be comfortable doing in that room, then, go from there.
For me, I usually ask, "Where is this room relative to our common areas(den/kitchen/front door)?" I ask because I don't like to do projects in a room that doesn't have a view of our common areas. When I'm "purposing" a room, I like to ask myself whether I'll actually USE it for that purpose. If I feel cut-off from the rest of the house in a particular room, it's best use will probably be for sleeping or, maybe, reading or laundry! For example, I know I won't use certain rooms for bill paying regardless of how nice a set-up I give myself in those rooms.
Any of the ideas from other people (above) would be good--IF you can see yourself being comfy using your room one of those ways. The migraine room is particularly creative!!
I used to suffer migraines until my doctor put me on a blood pressure medication, so, I can truly relate!