I think having a public display can be both inviting and not so inviting. If you come in and there's already a host of people already engaged in an activity, anyone with a sense of morals is probably not going to inject themselves into what's happening under the fear of rejection. For new people who have no clue what's going on, throwing them into a giant crowd tends to make them lurk and not interact. There has to be welcome gates for those that don't do well in a public forum. If I come in for instance and see that two people are smutting, if I appear into the mix and make myself known, the best possible scenario is that someone responds to me (every 4chan'ers dream, really). Asides from that, I could piss everyone off for getting in the way of their scene for putting a dick in an ass that didn't want a dick in an ass, or otherwise be ignored because the current group is invested in what it's doing and including another random person doesn't jive. These kinds of 'bubbles' that will form will end up behaving like those private rooms you're worried about whether you like it or not. The only difference is how public their privacy is.
WIthout the feeling that you can join that conversation, people will talk less, and trust me when I say that population is probably something you're looking for.
If creating a 4chan based public thread system is your goal, then there has to be some kind of funnel for newer people to go to. To grow, you have to create an environment where they feel like they can get started in the way you want them to. If you want them to start smutting right away, create a channel where you will primarily see newer people with a single objective of getting them oriented. Since you're gonna get all kinds of wacky people with all their wacky preferences and fetishes. I'm crazy in the sense that I prefer long-term RP with dice and a grand adventure in mind, and no one wanting a simple "let's have a hot fuck in our apartment" kind of scene is gonna get along well with me.
I'd say at the very least, creating those categories is your go to. So that in the spirit of 4chan, you match the "Epic Adventure Fags" like myself with similar minded people, and the "Quick Fuck Fags" with each other as well. If they're all in the same room it can get pretty wild to keep track of the convo, and you'll notice that despite the anonymity, it's easy to tell it's just a small handful of 4chan'ers having a conversation anyway.
TL;DR of this idea is, there's gotta be some kinda way a person coming into avanon can always get involved as much as possible. And a large, involved scene is hard to get a leg in, which can stagnate the population growth or worse, result in the abandonment of the project.