Instead of posting a topic in a subforum, what if subforums were turned into tags and you just post your topic globally with those tags. Now you can have a unified UI that shows all topics, and people can filter by tag.
I experimented with this with a /topics page that implemented such a UI. What I found was that it becomes one big soup that lacks the visceral structure that I quickly found to be valuable once it was missing.
There is some value to "Okay, I clicked into the WebDesign subforum and I know the norms here and the people who regularly post here. If I post a topic, I know who is likely to reply. I've learned the kind of topics that people like to discuss here which is a little different than this other microclimate in the RubyOnRails subforum. I know the topics that already exist in this subforum and I have a feel for it because it's separate from the top-level firehose of discussion."
I think something similar happens with modules and grouping like-things into the same file. Microclimates and micronorms emerge that are often useful for wrapping your brain around a subsystem, contributing to it, and extending it. Even if the norms and character change between files and modules, it's useful that there are norms and character when it comes to understanding what the local objective is and how it's trying to solve it.
Like a subforum, you also get to break down the project management side of things into manageable chunks without everything always existing at a top organizational level.