[wwbtszqt] the phantom menace

first, throw away most Star Wars mythology.  then, substitute our own:

Sith always operate in the background, manipulating from afar.  avoiding discovery is their highest priority.  (left to be explained is why.)  Sith Illuminati.

Darth Vader is a Dark Jedi, not a Sith, which is why he ostentatiously shows off his powers.

the Emperor is a plain old civilian, not a Sith, whom Vader respects and trusts for his leadership and political skills.  (previously similar.)  (or, maybe for the Emperor's engineering or engineering management skills.)  the Emperor's goals happen to align well the Vader's (and perhaps with the will of the Dark Side of the Force), so they are allies.

Vader fell to the Dark Side entirely on his own accord.  (this makes his story more tragic.)

back to the Sith: they are so good at remaining concealed that everyone, including the Jedi, wonder if they even exist.  and if they do exist, some wonder if they are even evil.  Old Republic humor, before the rise of the Emperor: "if they are secretly manipulating the fate of the galaxy, they seem to be doing a pretty good job at it: the galaxy seems to be functioning all right."

the Sith follow the Rule of Two purely as a practical method to minimize discovery: the fewer people that know a secret, the better.  two is the minimum necessary to continue the line, to pass along knowledge.

the Sith in fact avoid learning skills that ostentatiously show off the Force, e.g., they avoid knowing lightsaber combat, telekinesis, Force lightning, again in order avoid being discovered.  if a Sith gets into a direct confrontation with a Jedi, the Jedi anticlimactically quickly wins.  but it is unlikely that the Jedi even perceives having killed a Sith, as opposed to having killed a non-Force-sensitive muggle.

Sith plan their confrontations with Jedi to be indirect, attempting to kill them from afar.

humor: a Sith holocron is discovered.  instead of containing knowledge of things Sith do, it boringly contains a list of things not to do, how to avoid calling attention to yourself.

perhaps from the point of view of some characters, Darth Vader falling to the Dark Side and the galaxy descending into turmoil are not that big of a deal compared to discovering the secret of the Sith.  (imagine Fox Mulder in the Star Wars universe.)  perhaps everything is just part of a plan to draw them out.