I suggest you:
1. Read some books.
2. Watch some movies.
3. Take a stroll outside in the city.
4. Don't force yourself to think up an idea.
I often find inspiration coming when I least suspect it. For example, during my exam Dutch I got some nice idea's. Or while watching a movie I though up on a nice setting for a map. Often, combining these idea's greatly helps making a map unique.
Store up on idea's, make sketches so you don't loose these idea's. At this moment, I have several scenario's to try out. Too bad I can only handle one or two at a time...