Intellectual Property, of course.
Intellectual Property is the "industry term" for a coherent, usually copyrighted and/or trademarked group of concepts. IE, a universe. Star Wars is an IP. Lord of the Rings is an IP. Dungeons and Dragons is an IP.
Now, here's a fun fact that you may not have known about: an IP has a specific range that it's designed for. That is, the writer intends to explore specific concepts, and therefore the universe he designs is built to do that.
It's easier to see in smaller IP. For example, the game Eternal Sonata is built to explore a few core concepts involving death, the search for happiness, and the way they intertwine.
If you were to try to write a fanfic in the same universe as Eternal Sonata, it would probably be based around the same concepts, just naturally. You could focus on other themes, like the nature of art or insanity, but it would feel strange, kind of forced.
It would be like creating a Star Wars game about trading stocks and bonds. I mean, you COULD, but it doesn't really fit the IP very well.
Some IP are very restricted, to the point where all of the themes are explored. There isn't even any urge to expand on it (with fanfic or sequels). Not because it's bad, but because it's said all it has to say. There are lots of examples of this, mostly movies. Movies tend to be tightly woven, with no excess fat. Nothing to grab ahold of and run with. Some of these movies get sequels ANYWAY...
An easy example of this is the Blade Runner movie. If you see "Blade Runner 2", you know there's something dumb happening.
On the other hand, some IP are "built to last". That is, they are built to explore very big themes in very diverse ways, so they aren't in danger of running out any time soon. A lot of TV series are built this way.
Firefly, for example, is an IP that can explore a wide range of themes without being in any danger of running dry. Firefly was on purpose. An example of an accidental "built to last" IP is the Terminator franchise.
Some IP are "forced" into lasting. By which I mean they originally have very little to say, but they are stretched and deformed into an almost unrecognizable form that can explore a wide variety of themes.
Star Wars is an excellent example of this: the original movie was pretty strictly limited, thematically. Especially since the universe was so absolute: there are NO OTHER JEDI, there is EXACTLY ONE OTHER SKYWALKER, there are NO OTHER SIGNIFICANT GOVERNMENTS, it's just the Empire and the Republic... the whole fiction was built to serve this one exploration.
However, as time wore on, Star Wars was hijacked. The canon was weakened and expanded so that virtually every imaginable theme can be explored in a Star Wars setting without seeming out of place. I think you could probably make a game entirely about herding bantha and it would seem to "fit", so long as you had a light saber.
It's not that one type is better than another. Tight IP can produce extremely profound stories that are drenched in mood. Blade Runner is an example. It's a very strong, very powerful movie. If the universe had been weakened to allow for future exploration of other themes, it wouldn't have been as tight. Now, you could write Blade Runner fanfic on other themes (I'm sure it exists), but you'll be fighting the current.
On the other hand, a "built to last" IP can produce forever. While any given exploration of a theme might not be quite as tight, you can explore the whole of the human condition in bite-sized chunks. And, as Firefly shows us, "not as tight" doesn't equal "bad".
Lastly, letting your fans hijack your IP and turn it into a universe capable of supporting life is a fantastic crossbreed of the two other types. The downside being, of course, that anything you make cannot hope to live up to expectations... ;)
Anyway, that's all kind of tongue in cheek. The real point is this:
Game rules are the same way.
Some game mechanics (rules and progressions and so forth) are built to explore a specific theme. The really obvious examples are indie games like Passage, but a less indie example would be Eternal Darkness.
Some game mechanics are built to support a wide variety of themes. An example here is the first-person shooter, which can support a huge number of themes, only a handful of which are actively being explored. Sure, most of these themes are rather physical, not all high-falutin'. But they're still themes.
Some game mechanics are hijacked and forced to deal with a huge variety of themes without their consent. A good example of this is D&D and it's ten trillion descendants. I don't think I've seen a genre that doesn't have an example of a D&D-like system.
Game rules are just as much a way of expressing things as pictures or dialog. Different things, but things nonetheless.
I'd like to see some games that have an IP that is equal parts game dynamics and universe design... working together on exploring the same themes.
What do you all think?