Star Trek would be great. As with the Lego Star Wars theme, they could have large and miniature sets of Enterprise, Klingon Bird-of-Prey, Romulan Warbird, etc.
Then you could have sets from the other series (Next Generation, DS9, Voyager, and Enterprise) and of course the movies. The possibilities are endless.
To have a mini-figure of Worf would be beyond awesome!