Instead of going for a single conventional theme, maybe mix themes together for interesting results?

For example, something like dark underground dungeon could be combined with modern elements, instead of being totally medieval. Such a theme would require more creativity.

Also Factory?

There are many styles to make a factory with. In fact, it's possible to make a versatile set that could be used for factories, spaceports, buildings or other similar structures. In much the same way a set like Top Secret III has great versatility. Although I brought up in the past that styled sets are better for themed levels, because you won't run out of space for tiles as easily.

My vote: Other