To start out, I need to comment that we could measure how unrealistic any fictonal setting is by counting how many ways it is different from real life "just because". With this metric, only axioms are tallied, not their consequences.
For example, the Star Wars setting has, among other things, the force, lightsabers, faster-than-light travel and communication, and tractor beams. None of these are physically possible, and none are natural consequences of each other. In contrast, spaceships and aliens are futuristic but in a general sense physically possible.
Now, this is not a universally useful measure of how unrealistic a setting is. Fans of Firefly might ponder and/or appreciate how firearm technology has been almost stangnant for 500 years in that show's setting (in the year 2517 people are almost exclusively using today's guns). But my metric ignores this issue.
Nevertheless, for a role-playing game, this metric is very important.
The setting of a role-playing game needs to be intuitively understandable to the players, so they can estimate what are wise or foolish choices. This is different than settings for a novel or movie, in which some aspects of the setting can be left unexplained. In the Star Wars setting, those watching the movie do not need to understand how the force works because they do not need to make prudent decisions about when and how a character will use it.
One reason I am designing a role-playing game, as opposed to using a published one, is that the published games tend to be limiting in artificial ways that frustrate intelligent players.
Typically too many questions are unanswered, in one of two categories:
- The game setting does not allow characters to even attempt to use various skills or objects. Or the game setting is that flexible, but the game mechanics do not specify what happens when skills or objects are used in unexpected ways. (For example, there are poisonous plants but the game's setting or mechanics does not allow making poisons out of them.)
- Something (the force, laser weapons, magic, super-powers, etc.) works according to a rather haphazard collection of rules. It is impossible to modify these rules, even in ways the rules would find self-consistent. (For example, laser pistols must be getting a lot of energy from some sort of battery, but the game's setting or mechanics does not allow dismantling laser pistols to make use of their batteries, nor equipping the laser pistols with high-powered tactical spotlights.)
To keep the rulebook brief, the setting must be intuitive. To maximize this, I am designing a science-fiction setting with only one impossible feature (and its logical consequences).