Aliens, the supernatural, the paranormal...some of the most intriguing mysteries and unanswered questions of our existence deal with these three fascinating subjects. I have read volumes on these topics and still want to know more. The X-files was a great show and MIB was a great movie, but what really did it for me was the game X-COM. It really brought the genre into a great context, in my opinion. Though it mostly dealt with aliens, there was plenty of room for expansion. The idea of a paramilitary group that acts as both protector and investigator was awesome and would work extremely well in a tabletop rpg.
The first thing that had to change though was the thought of the government cover up. It had been done to death and was really cliche at that point. So I brought these strange beings out into the light and into the open. It was bound to happen sooner or later anyway. That way they could keep people informed and protected. Plus the genre always seemed overly dark, since many of the creatures could easily fit into a horror film. But I always found that the stories were more interesting when they leaned away from the blood-and-guts of the horror genre. Supernatural tales had more depth when they were based on the Bible or other myths. Aliens were more intriguing when they could actually communicate, and so on.
Since the stories were going to be very rich and mysterious, the system need not be overly complicated. One die and a chart was all I needed to get it rolling, so to speak. No reason for the d12 though. I just like them. I was surprised at how quickly and easily the mechanics and the game itself all came together. It was only my second game of course so I had yet to realize that rules didnt need to be too complex to be good and fun. Since the characters were basically volunteers, normal folks from all over, just like you and me, I didnt feel the need to bog them down with a lot of weird skills and abilities. Just your average people trying to do an extraordinary job. It wasnt until later that I hit upon the idea to let players take the role of non-humans. Why not? There could be traitors in the alien ranks or paranormal sympathizers. Plus the development of psychic powers seemed like a logical step, especially after fiddling with all that alien tech and supernatural ectoplasm.
Still, I think my favorite Earth Threat mechanics revolve around the experience system. It was broken down into separate categories to encourage more than just hacking the enemy to pieces. Those amounts multiplied together and added to the number of missions you had undertaken made for a perfect promotional platform to rise within the ranks of the organization. And Rank was definitely a good thing, since level-based characters never really felt truly appropriate in anything but a fantasy game, for some reason. Maybe it's just because D&D did it first. Who knows.
I think this is a great take on a niche genre, with some added value combined in and a few random elements to keep it fresh. You can do a lot with it. Hope you have as much fun with it as I have!