A thing that can be called "model" is forming in my mind for some time, but I have a very hard tim to implement it or even to phrase it. Let's give it a try to have a snapshot of this thought.
This is the seed of a model of classic RPG (one almighty GM, more players with one PC each).
Flow of Information
As I see it role-playing isn't anything else than communication between the players and the GM. Certainly the interesting link is between GM and player, because, the games are set up to handle this and no player-player communications.
The GM is steering the game by adding informations and listening to the reactions of the players. The fact that every player has one character wich is there to channel the players' decisions makes it somewhat easier for the GM. The PCs structure the information submitted by their players. The information submitted by the players are more structured than the GM's.
But even the information submitted by the GM has to be structured. It is not mandatory for the GM to have something that structures this information, but it isn't mandatory to do everything on the fly either. So, the GM can prepare a document to help him/her in the structuring of the information - this document is called the adventure. You can notice, that even in freeform games the GM always starts to GM in some pattern that resembles an adventure. Because if the information submitted by the GM wouldn't be structured, it would be meaningless.
My point is: actual play is nothing else, than this two-way flow of information.
Limited Aesthetics and Decisions
In actual play the sole role of a player is to make decisions. Even the description of the PC is only a decision how the player chooses to dress his/her character. The player won't describe anything if it has no impact on the "story". This is the difference between the GM and the player: the GM mainly describes, but the player only and exclusively makes decisions. Even if it is supposed to be mechanical: the player rolls dice for his/her character to have the character's fate in his/her hands.
The purpose of the GM is to give choices and opportunities for decisions. An adventure should be a string of decisions by the players even if they don't know that those are decisions.
But I propose that there is another element, namely aesthetics. It is an important fact, that the actual play isn't written, but only live and thatswhy the memory of the players limit the extent and length of information stored, not to speak about the stress caused by the situations of decision. But I propose, that there is enough capacity left to appreciate the structure of the story on some levels.
My point is: the main portion of play consists of the decisions of the players in the situations presented by the GM.
My other point is: There is always an adventure prepared or not, only the structuredness of it changes.
Smaller World than Setting
There is always a self-contained adventure written based on the setting. It is impossible for the adventure to cover the whole setting, so the world of the adventure has to be smaller than the world of the setting.
This means that the adventure is a whole seperate thing from the setting. An adventure should work in itself. If I take away your sourcebooks, you should be able to GM your adventure from your GM's notes. If you have not sufficient GM notes to do this, than you probably have to rely on your adlibbing even with sourcebooks, no matter how accurate and handy they are.
My point: The adventure is a self-contained system that consists of a string of decisionmaking situtations for the players.
My other point: the adventure is written for the players by the GM not for the characters (and not by the arch-enemy).
Functions
But how does the adventure impact the players? The adventure is nothing else, than a gunship carrying impacts instead of guns. The adventure has no other purpose than to have impacts on the players.
An impact can be for example what an NPC does to a PC. Thatswhy I call this impacts "functions" as Propp did. (Although maybe it would be wiser to call these impacts.) There are two kinds of function: game function and dramatic function. (Maybe I should call game function function and dramatic function dramatic impact.) The kinds and types of game functions are in your rulebooks. For example it can be combat or magic in a classic fantasy RPG, or it can be skill use. Anything that is covered in the rules. The dramatic function can be anything else wich is possible in the setting. For example talking to an NPC or the changing of the weather, if it isn't covered by the rules.
There are carriers of these functions. The most common carriers are NPCs, but it can be the nature or just a "random" event. Note, that the function is more important than its carrier. If you want to injure the characters, you can do that by combat using NPCs or by a fall from a high place. It's the same function, but a different carrier. And note, that the term "random" is between quotation marks. That means, that real randomness has no sense in this case. The adventure is there to structure the gameplay and randomization would complicate things not structure them.
There are other distinctions between sets of functions. For example a function can have the purpose to establish something or to impresse the players. Impressing the players always works as a counterpoint to the establishment. The conflict arise only from establishments and their counterpoints, but the establishments themselves can serve as situations of decision.
My point: Only conflict is interesting, so you always need a counterpoint.
Note: Carrier can be defined more loosely. The sentences of a GM carry the functions of the adventure. Using this definition can help making rules about text and can have dynamics of text like you would have in a novel or a drama.
Levels of Player Influence
Remember: This is about classic role-playing games with an almighty GM and long skill lists and long discussions in critical moments about who is where and does exactly what.
There are different levels of player influence in different types of content.
Player decisions are 100% player influenced.
GM secrets are 0% player influenced. (These are there to be uncovered by the players.)
There are "technical" facts in wich specialist players can make suggestions for the GM. Like lasers in a sci-fi or weapons in a fantasy game.
There are sole-player issues where the player and the GM are refining the facts, for example the alternatives and neighbours of a PC.
There are party issues which have impact on the adventure itself. Supporting NPCs and extras can be made up by the players and approved by the GM. This helps the GM as (s)he don't have to come up with everything.
This last one is still only technical stuff, I have no important point yet.
Enlightening!
Usually I type replies rivalling novels in their length, but unfortunately, I have not the time nor the bandwidth to do so right now. What I would like to say, though, is that your article really opened up a lot of ideas for me. I find it and intriguing and refreshing way to look at the structure of a traditional RPG, and it illuminated a few key elements that I have been missing for a long time in making a successful game that is enjoyable to play.
There are a few points I woudl like to interject with, if you don't mind. I cannot completely agree that a game, at its core, exists only as a channel for decision making and challenge resolution. I believe that within a game is a "spirit", you might say. There are many games out there that are structured and mechanically sound, yet some are enjoyable, while some seem like a burden to play. There must be something beyond the core aspects of the game that creates a sense of excitement in the player. As it is with all things, I really see the game to be greater than the sum of its parts, even though an understanding and intimacy with said parts is crucial, as well.