One of the most interesting (and perhaps frustrating) things introduced was the Sith 'Rule of Two'. As put in the movies: "There are always two, master and apprentice." This was later expanded upon as supposedly a law set down 1000 years prior to the movies. That the number of Sith would be kept to only two, to avoid the terrible conflicts and infighting that always tore the Sith Order apart.
Right away, however, we are shown that this isn't the case. Darth Sidious is the master, yes, but in the first movie, Maul is his apprentice and in the second, it is Count Dooku. Obviously, Dooku existed at the same time that Maul did—and its reasonable to assume he had fallen (or was in the process of falling) to the darkside at the time. And there had to be some kind of training program in the works. Otherwise, when Sidious loses an apprentice, he'd have to start all over from scratch. That just doesn't make sense. So, to me, the 'Rule of Two' means not that there are only two sith at a time, but that there are only two 'top dogs' in the organization. It stands to reason that there are dozens, if not hundreds or even thousands, of 'hopefuls' and disciples to the Sith Order. To believe otherwise makes the position of 'Dark Lord of the Sith', rather a hollow one. Number two man in an organization of two guys.
In the original (2D) animated Clone Wars series, this idea of multiple 'hopefuls' certainly seemed to be the case. Asajj Ventress was introduced as an apprentice to Count Dooku. She considered herself a Sith (though she was 'corrected' by Dooku in this, rather violently). But whatever the case, it shows that even in official canon (which the cartoons supposedly are), there are minor Sith out there, waiting in the wings for their chance to seize power. The concept of Emperor's Hands (such as Mara Jade) seems to support this idea as well—as they seem to have had force powers in addition to other skills.
Therefore, in my gaming universe, there were a number of lesser powered dark side folks, most of whom were associated with the Sith Order. The most prominent of these was Lord Stromm (nevermind how pathetic he turned out to be...), but there were others from time to time as well (the foppish, narcissistic Lord Qar comes to mind here). Not only are these 'minor sith' a reasonable assumption, they are necessary for a Star Wars campaign. They provide good, scalable foes for a party—capturing the 'feel' of the movies without having to have Darth Vader as your arch nemesis.
In the movies and subsequent comic and novel series, we are led to believe that Yoda, Obi-Wan and Luke (and later Leia) are the only Jedi left. Again, I find this a bit difficult to believe. There were likely THOUSANDS of Jedi throughout the galaxy. I am sure that the bulk of them could have been killed during Order 66—and many more were likely hunted down in the 20 years between the trilogies. But the idea that only these three (of four) remained is a bit much to believe. I'm willing to bet that many Jedi survived—though perhaps none quite as powerful as Yoda or Obi-Wan. Likewise, I'm sure there were new Force users being born ever day—and even with the Empire hunting them down, the Galaxy is a HUGE place, with lots of remote corners to hide in. Hell, if the Empire couldn't find Luke SKYWALKER—who didn't even bother changing his last name...which makes you wonder if Skywalker is a common name.. the 'smith' of the Star Wars universe...
Again, the idea of many minor Jedi and Force adepts is a central assumption of the roleplaying game. The big challenge of the GM in this regard (at least in my opinion) is to see that these PC Jedi don't steal the stage from Luke. But that's just my personal preference. As much as possible, I do not like to detract from ANYTHING presented in the Original Trilogy. To me, to do so is to make it not Star Wars (the prequels? Well.. I play a bit more fast and loose with them).
Anyway, that's it for my ramblings for now.