Go go five-minute game!
Premise: Jacobeans vs. Aliens
Reason: looking through game PDFs, and noticed that "secretly fight fantastical creatures in modern world" is a big trope. Meandered around the thought a bit, via "could you reverse it?" (difficult) and "it's always using new stuff to fight basically old stuff", to "history vs. sci-fi". Then there came a touch of "what would a sci-fi game look like to people a few centuries ago?"
It's the Jacobean Age, at least in Merrie England. Astronomers and astrologers alike are astonished by the sudden appearance of many new stars and meteors. They are even more astonished when these start falling to Earth, revealing themselves to be no meteors, but vessels bearing beings from the outer spheres of the cosmos! And naturally, they do not come in peace.
The allospheric beings are soon attempting to gain a foothold on the earth. Their great aetherships of iron, copper and sphere-crystal descent from the heavens en masse or in secret, trying now by force, now by guile to overcome the stout folk of Earth. Now let the King's champions take up arms to drive the invaders from this peaceful realm.
Strictly speaking, having the ability to invade other worlds involves fabulous technological advances that would, in passing, render the arms and tactics of the Jacobeans worthless. But let's pretend we don't know that. Maybe the allospherics simply never needed to be any good at fighting before, so they only really have quite primitive armaments. Maybe the aetherships work on such different principles that they didn't lead to the development of rockets, internal combustion engines or atmospheric craft. The important thing here is that, while (as always) the aliens need to be presented as having super-advanced technologies, mechanically they can be fought effectively with horses, black-powder weapons and swords. The technology's more advanced all right, but that doesn't make it stronger.
Otherwise, this will work much like other types of urban fantasy games. Sometimes the PCs need to root out an alien infestation in a small town, sometimes to hunt down an obvious invader, and sometimes to aid in a straightforward fight. The historical setting means information travels slowly, making it easier to support hidden alien sites and public ignorance of what's happening. The aliens can be a secret known only to a few, or a public threat, as the gaming group chooses.
I've been listening to a lot of Hunter recently, so I'm going to say this game uses one of those Storytell* systems with d10s. Maybe I'll incorporate Arthur's idea about having Skills be worth three times what Attributes do. Not sure. Either way, it's classless and skill-based.
There'll be a definite tradeoff in equipment, specifically bows vs. firearms. Strung weapons are broadly more reliable, quiet and faster to use. Black powder weapons can be fired one-handed, used readily in closer quarters, and need less training - but tend to misfire.
Alternative title: Get with child a mandrake root