There is already possible to create a topic in Questions to group threads. It would be really useful to enable creating subtopics for topics.
e.x.: There is a `house` topic and I'd like to create a subtopic for it like: `roof` and `walls`. As the result in `house` there are all topics from `house`, `roof`, `walls` topics and also `roof` and `walls` have the same followers as `house` (automatically inherited from `house` after creation). Now I can add separately guys as followers for `roof` topic who don't care about `walls`.
Moreover, it would be nice to create a few levels: `house` <- `roof` <- `bricks`