My inclination here is, if we really want to do this right, we should do this correct asap, and not just patch things up.

Solution #4 by fatalsaint or #6 by Dave I think are the best ones as they lets you target the the specific platform on any discussion. Plus, searching will be easier since it is easier to filter a search based on a specific platform (parent forum category).

If we implement solution 4 or 6 however, there will be a massive move of older threads to bring them to the correct platform, under the same category. Although tedious, it can be done.