I look to books, history and movies for most of my wargame scenarios. I also like to link them together if I can. The book The War of the Worlds, by HG Wells describes a Martian invasion of
Each of these stories alludes or even depicts warfare against Martians in many parts of the world. Since my wargame collection includes the US Army since the beginning of the nation, I can play US Army vs Martians scenarios based on the depictions in these stories. I use walker Martian war machines for at least the 1898 and 1935 Martian attacks. For the 1953 attack I can use the Pegasus Martian war machine. For the 2005 Martian war I can use the new Pegasus Martian war machines that should be out very soon.
The troops that I use for the Boxer Rebellion, the HaT British Colonial and Americans I can use for the 1898 invasion. I can use the HaT & Imex WWI US Army troops for the 1935 invasion. My Imex Korean War troops work well for the 1953 invasion. For the most recent 2005 version, I can use the new Caesar Miniatures US Army.
Playing a science fiction game with historical figures allows me to get double duty out of my American Army. It also allows me to play another period, with an investment on only one side of the conflict. I am also using the Twilight Creations Martians! figures for Martian infantry. They are similar to the Martian soldiers in the Mars Attacks! movie. Twilight Creations says their Martians should be available soon as separate figures.
The other advantage of science fiction wargaming with historical figures is the terrain gets to do double duty. I already have American civilian buildings for use in historical wargames. So the cost of terrain is already paid for to do science fiction gaming with historical figures. Historical sci fi gaming is a good use of your collection.