Forum Organization Shared Practices

I have a "shared practice" type question and wanted to get thoughts from my fellow community architects using Telligent!

We're creating a group called "Questions & Discussions" that house most of our forums.  We want it to be really easy for users to navigate between the different forums, but make it feel like they're staying in the same place... creating a seamless UX for all questions.

 I'd like them to see a listing of all recent forum threads and be able to use the sort/filter tools to look through existing questions. I'm guessing the best way to do this is by using the Forum - Thread List widget. Then, on the right side (1/3) of the screen I'd like to see a list of our (15) Main Topics... similar to what Appian has done (image, right)

Now here's the caveat... Each of our Main Topics have 5-15 different sub-topics or categories. I would like to have this list on the right to function in a similar way to what Appian has done, so when a viewer clicks on one Main Topic, it would go to that forum AND show a list of all the sub-topics or categories within each main topic. SEE ATTACHED PDF.

So I was thinking that the best way to do this would be to create sub-groups for each of our Main Topics, beneath our ROOT "Questions & Discussions" Group. Then, we could create as many forums as we need to represent each of the categories within our subgroups.

I can envision so many ways to do this but I'd love to have a discussion with others to see what they would do, and why!

Some specific questions:

  1. We'd like to allow users to subscribe to individual forums, so I was thinking we would need to have them all as sub-groups. Would we need to have all of the granular categories within a Main Topic (Sub-group) as another sub-group??
  2. We'd like to have it so when a person clicks on one of the categories/main topics, on the right, that it opens to reveal all of the sub-groups or forums (depending on which we end up doing). What considerations should we take into account in order to be able to do this? Architecturally, would we need to use sub-groups vs. forums to be able to show all of the sub-topics?

Here's a sketch of what I was thinking of doing...