I don't know if I'll actually run this game or not, but I am feeling inspired by this idea. If I don't run it, maybe it will give somebody else an idea.
The first idea was to run a "17th Century Pathfinder Game", which I described as "Three Musketeers and Zorro with Wizards and Witches". Then I was talking to a friend about time travel games and started thinking "What if Rene Descartes had designed a working time machine and Queen Christina had built it?"
So, my thinking shifted to the 17th Century, still with Musketeers and masked "Zorro-types", but with a storyline that could involve Queen Christina of Sweden and time travel.
I just started thinking about this a couple of days ago, and need to get to the library today to see if I can find a book about Queen Christina of Sweden.
On the Pathfinder aspect:
P1. Actual Pathfinder characters are very rare. There are Witches and Wizards, but not enough to have really influenced history. This is a low magic campaign.
On the Historical aspect:
H1. I plan to stick close to history as recorded, with maybe a few minor changes here and there. (I am considering delaying Christina's abdication for a few years, for example.)
H2. There is the potential for the PCs to alter history. History has an inertia that tends to keep it in the same direction, but a push in the right direction can make a significant difference.
H3. I have set the start of the campaign in 1653, shortly after Descartes's death.
H4. The time machine was not completed until after Descartes's death.
On the Time Machine aspect:
T1. The time machine is colossal and immobile, being anchored to the Swedish bedrock. It is literally a building, a big building. Most people do not know that it is a time machine, thinking that it is a new palace under construction, and the queen controls who uses it.
T2. The machine can also go to alternate worlds. Thus, it is possible to have adventures on Golaron or in the Forgotten Realms or any other setting I might want to use. This is one thing I like about this setting.
T3. The other planets in the solar system are inhabitable, somewhat like in Victorian science fiction. (Thus, I can implement some of my ideas for a "Space 1699" idea.)
T4. The time machine can not go to any time before it was built. Thus, Queen Christiana can not use it to prevent Descartes's death. Bummer.
T5. There may be one or two other time machines in existence. I am considering putting a time machine in the Taj Mahal, for example.
I have to get off to the library to do research, but I think Pathfinder has a lot of aspects that will support my ideas. As for the campaign:
C1. I'll probably start the PCs off at 1st level, and not associated with Queen Christina.
C2. An early adventure (perhaps the first) may include catching a burglar who, among other things, has stolen Rene Descartes's skull.
C3. After recovering the skull, this gets Queen Christina's attention, because she wants to know why anybody possesses Descartes's skull in the first place.
C4. After solving that mystery for her, Christina may give the PCs more adventures. Early ones are more likely to involve travel around the 17th Century world. Eventually she may send them on time travel and/or off world adventures.