This is a suggestion for a different way to organise forum threads.
Each top-level post starts a new topic. The author picks a subject line for the topic. Descendents of the post that started the topic don't have subject lines of their own.
Within each forum, we show a list of topics. These can be sorted by recent activity, so inactive topics fall to the bottom. Subject lines provide a summary of the topics currently being discussed. The user can open an existing topic or start a new topic. Within each topic we show a threaded view like the one we currently use for the forum as a whole.
The aim is to allow parallel conversations to happen within a single forum, while making it easy to navigate between different conversations or focus on the most interesting ones. Subject lines make it easy to collapse inactive conversations down to a summary.
The main disadvantage is adding another level of navigation. The distinction between the forum list and the topic list might not be clear.