Oooo I hate open ended assignments too. I've managed to avoid those this year.
Scifi is not my specialty (I'm more of a dark fantasy/end of the world scifi type girl) but I found these themes listed on a website:
Speculative Science Fiction
Space Opera
Hard Science Fiction
Temporal/Alternate Dimension
Media Tie-Ins
I don't know if I'm allowed to link to the site, but it has some good stuff. Just google
themes in science fiction. It's the first hit. Might give you some ideas on what to write
Even gives authors in each genre.