Just a thought, muskets were innaccurate over 50 yards, and the standard procedure was to stand in a line, and fight from within that position. Just how entertaining might that be in an FPS?

As for WW1, perhaps aiming might be more worthwhile, however, I'm not sure how much fun climbing out of a trench and walking 200 yards into a wall of lead might be...

Basically, the reason most FPSs are based on a more modern period is that as time has gone on, engagements have placed more weight on individuals, rather than units. Vice versa - goes part way to explain why strategy games more prevalently depict periods farther back in time.

For an entertaining (if very old) Napoleonic RTS, try Micropose's Fields of Glory...