RP / Collaborative Writing

I'm not entirely sure what you mean by 'Collaborative Fiction', not having taken part, so I can't outline the differences very well, but here are my thoughts. All this- it applies here. Other places may do things differently, I don't know.

I would say that- generally speaking, the creator of the thread is more responsible for the setting of the thread than for the content of the thread, or "twists." (Under the understanding that a thread in the Overworld represents a location in the game world.) What it does, how it might be affected by actions characters take, and things such as that.

For example, one of my more popular threads is The Midnight Sun. It's essentially a combination bar and tavern in Isra, and the thread represents that physical location. As it's my thread, I'm responsible for the setting there. For the barkeeps and the innkeeper, for the drunkards and miscellaneous staff members. I have an alternate account, The Midnight Sun, that I use to represent anything having to do with the place.

I'm also responsible for Isra as a whole- Isra's a nation, an empire, and a city. I use my account, The Isran Empire, to represent things like guards, soldiers, the populace in a general sense, and to create locations in Isra. This Central Plaza, for instance. In the case of large areas governed by a group, a nation, the player responsible for the nation as a whole also has a degree of control over what threads other people make in it. 

For example, Renalt Fletcher has created this thread, entitled Fletcher's Delectables. It's a candy shop that he runs, in the city of Isra. Many other characters also have homes and shops inside the city, and each of those threads has my approval as a valid setting in the city. A thread representing a mountain, for instance, would not go inside the city.

But the basis is as such. Threads represent locations. Sub-boards (such as this one which is the city of Isra) represent an area in which other threads would be grouped in a logical way. Like a city, a castle, a nation's lands, a province, or some other grouping like that. Some players choose to act as 'GMs' and create threads in which they function in that manner. The best example of this is Strangers and Travelers, an account that represents whatever exists in any of the threads they've made. When a character goes to an S&T thread, they can expect the setting there to be interactive and intending to express some sort of narrative. 

Not all threads are necessarily interactive, though. This thread, the Lavender Hills, are just literally hills covered in lavender flowers. No one is there, there's nothing there in particular to do besides interacting with other characters- which you can see there. I'm not going to post as that setting because there's no reason I would need to. I created the thread as a location to be used as part of some things that didn't pan out in the end. Now, it's just a place that characters can go, and I'm not responsible for creating any sort of story for them.

Other threads are like that as well. The Saucy Maid Tavern, for instance, is used as just a general purpose tavern location, where the characters visiting can assume that the tavern functions, and serves them drinks and such. The Midnight Sun can work in that way as well if the character would like, because in some cases, the posts aren't about the location very much at all, and are only used as a place in which to interact with other characters.

And all of that varies by the thread. A reasonable way to guess would be to consider if the setting has some active element that would be relevant to the character visiting, like the owner of a store, perhaps. Threads created by players who have since stopped playing- you're not going to get a response there either.

But speaking in a general sense, I wouldn't necessarily say that the thread creator is expected to "drive" the RP or function as the GM for characters. They're more responsible for acting as whatever the thread represents, and it's usually up to the characters to make a story within the world at large.

There are also quests, in which the quest creator may act as a GM. There's the Tower of Eons, which I'll just generalize and call a dungeon. It has this listing on the Quests board. The Tower's account acts as a GM for its thread, and within that thread, the setting- the tower, is actively affecting characters, with traps and monsters and stuff.

In a separate category, there are also what I would call advertisements on the Quests Board, like this one. In that, I wasn't really offering to create much of a story for someone, but rather to enlist some character to go out and explore for me- places I don't own. There are also bounties, for example.

In reading back on the above, I realize this is mostly rambling, and may not tell you what you wanted to know. If you'd like, please ask for clarification on something, or another question or however; I'll be happy to answer.

The line between RP and collaborative fiction is pretty blurry at the best of times, and around here, it's kind of all over the place on top of that. As a rule of thumb, the Open Lands and Open Seas tend to drift more into the "DM running an adventure" style more often, but that's not always the case. You also have several settings there that are basically proto-nations.
As a sandbox, you're technically kosher to show up anywhere you can justify turning up without breaking existing descriptions of the setting, but it's usually a good idea to read a bit of what's going on and if it looks more like a small group doing an adventure, message the people in it before you get in the middle. depending on what you mean by "twist," you might want to run that by people in a quest-type scenario first, too. If you're putting in a plot twist that the character you control is a spy or whatever, that's all you. If you want to have the house catch fire, that's on the thread owner, but you can totally throw torches at things and see how it plays out. If you want to say "oh, this place is haunted," that's the thread owner's call and there's not much you can do about it except suggest it as an idea. Me, I can haunt things, but I'm special that way.

To do something like that, you'd need to gather up a some players that were interested, and... actually, that's about it. If it worked out, at the end, you'd have a pretty solid base to drop an application to get the 12 kingdoms officially recognized as a nation with a government and go from there.

It would play out slightly differently, in that each player could only write their own characters, rather than having complete narrative control, which makes having something that big unlikely to work out very well. That many participants in a single scene would be a mess. And getting that many people on-board wouldn't be easy, especially right off the bat.