Slop.

I think if you want something without explicit communities centered around a singular main thread, I think something like a "tread tree", by which I mean you would have a main thread and users could either post in that thread or make a "node post" that when you click on it would take you to a sub-thread with the "node" as it's root, might be a better option than a single linear thread.

Not sure how one could go about integrating that system with the rest of the fediverse though. Maybe it could treat other instances as other trees where posts would be nodes of the main thread and comments would be in the corresponding sub-threads? You would simply have to sort the instance's posts chronologically to generate the main thread so it shouldn't be too hard to implement, and if there are too many you could only fetch the last few and only fetch older posts when the user scrolls far enough.

Also, without communities and with how long that system would make the main thread, that would definitely need some neat way to navigate the tree with the keyboard and some good filter and sorting options for users to get what they want from the tree, so tags, flairs, hashtags, and ways to look them up quickly would be a must as well.