I used a particular interactive model, which I will describe. However, all sorts of interactive web-forum ideas are possible. I will try to extract the useful lessons from my experience. Hopefully, they'll be of general value.
Here is the "Plum Lake Hilltop" game, in its completed form. (Here is the original forum thread.) Quick summary: I would post and describe where I was and what I saw. Then I'd ask for suggestions about what to do next. Forum readers would cheerfully jump in and post ideas. I'd pick a suggestion, post that I was doing it, and then describe the results. Repeat cycle.
I can comfortably say that this worked very well. The people who got involved say they liked it. The thread never got bogged down or abandoned; people posted nearly every day. (Every day but one, in fact.) I designed a bunch of material, and it all got found and/or solved.
What did I do to achieve this? What did I fail to do?
I wrote the first few area description in advance. I didn't really have to, because I'm comfortable writing descriptions on the fly -- as long as I have a very clear idea of what's there to describe. For most of the thread, I was writing the text as I went. But the map and item layout was (almost) completely fixed.
I kept the tone very conversational, and I wrote in first-person. (As opposed to the purely descriptive, mixed-POV "Linking" post.) I wanted to keep it clear that I was doing these things. This was not mere selfishness. I wanted to be able to say, "I'd rather not do that." I also didn't want people posting contradictory statements: "I do this", "I do that". That would have put me in the awkward position of telling someone he was flat-out wrong. By maintaining a monopoly on the "I", I was able to keep a consistent narrative.
(One player started with an "I'll do this..." suggestion, but got into step with me quickly.)
I tried to write everything in past-tense. That is, I maintained the fiction that I was going to Plum Lake, doing things, and then coming back home to post. This was harder than it sounds, and maybe not worth the effort.
(An example: I was careful to avoid statements like "Now I'm going to push the button..." It wouldn't make sense to find a button, link home, post, link back to Plum Lake, walk back to the button, and then push it. But I don't think anybody would have cared. People would have just gone along with the story. So I made my job harder than it had to be.)
Important lesson: people will misunderstand you. I've had a lot of practice at describing situations, and people still visualized things wrong. Be very clear; repeat important details; correct people's mistakes.
Repeat unimportant details, too. In a text medium, descriptive prose is everything. I posted one diagram, but mostly the words had to carry it. Looking back, I wish I'd been even more descriptive. I would mention the sound, smell, or color of something -- once. Then I'd fail to go back and mention them again. Those words create texture. You don't want to copy-and-paste phrases, of course, but you do want to keep them alive for the reader.
I tried to lay out the ground rules from the start. I didn't say "Hey, this is my thread, you can post too!" I posted a description and then asked, explicitly, "What should I do now?" In fact, my first question was even more straightforward: "Should I explore the crater or head towards those buildings?" That was pure pump-priming. I wanted to make sure somebody would post something, and the more interesting choices would come along later.
In retrospect, I should have been even more explicit. I designed a scenario which worked by the standard Myst rules: you are here, you are alone, you have nothing but what the Age provides. You can't run home for a rope or a wire-cutter or a grappling hook.
Really, of course, those rules make no sense in Uru. There's no in-character reason why you can't bring a flashlight to Eder Gira, or a ladder to Negilahn. We accept those restrictions because that's the way the game is.
The forum doesn't impose those restrictions, and a lot of the early suggestions blew right through them. I had to fill in all the ground rules with barely-in-character dictat: "I'm not carrying things into the Age. I can't carry much of anything up that hill. I'm not athletic. I'm not allowing anybody in to help me." Which demonstrates why I chose first-person POV! But I should have explained all that at the beginning.
The good news is that, again, players will happily play along -- once they understand the rules.
And they'll also play along with breaking accepted rules. You will often hear this: "Don't say anything that contradicts what people see on their screens." Plum Lake is an experiment in violating that. (In a controlled way!) I posted that I found a new Linking Book. I posted that I'd visited a new Age. I can't put that Age on your screen. But people were willing to go with it, because it was interesting and they felt involved.
(It's true that I restricted it to a forum and thread with the "fanfic" label. I didn't spend time in-Cavern talking about Plum Lake. Maybe I should have. Did it get discussed in any Bevin rap sessions? I am curious.)
The one ground rule I did explain early was that I'd accept exactly one suggestion at a time. This was important, because I'd already gotten two simultaneous suggestions that would both produce interesting results! I didn't want to follow them both in sequence. I would have had to put several actions occurring in one post. That would have been long and hard-to-follow; I was afraid that it would drive people away. But I didn't want any ideas to fall through the cracks either. So I said explicitly that people should feel free to re-post suggestions I'd skipped.
That worked out well. But as it turned out, "exactly one suggestion at a time" was too rigid. I wound up following this rule: "Answer all descriptive questions, and try all the suggestions that don't work. (Explaining why not.) Then follow exactly one suggestion with an interesting result."
I also intended to follow a strict post-wait-post-wait cycle. But in a few places, I diverged from that. One game element (the crystals) required observation over a long period of time. But I knew nobody was going to suggest "Do nothing!" So I posted an outcome, waited a bit, and then posted "I just noticed something odd..."
(This was a good way to turn a failed suggestion into a good one. Instead of saying "I can't do that to the crystals", I was able to turn it into this sort of double-post, with an observation.)
I also double-posted in a couple of cases where (a) there was no more to do in that area and (b) there was something obvious to do elsewhere. I waited a few hours for someone to say "Give up and try elsewhere," but after a certain point I just moved myself along.
I've got Plum Lake in my head. Some of you do too, now. Maybe someday I'll model it and put it into the Uru graphical engine. But I don't have those tools, and I may never develop the skills. I didn't wait. I opened the Age.
We think of the benefits that Age creation tools will confer. Those benefits are real. A forum post can't create the sense of presence that a 3D graphical environment evokes. But then, a 3D graphical environment isn't real life, either.
I've said that Age creation may become the true way for Uru players to tell Uru stories. Other people have said this too. I still think it's true. I also think that it's just as true for prose Ages as for 3D-modelled ones. They're not the same medium, but both can support ideas and experiences.
Choose your rules and go for it.