Hooray for forward-thinking
Initially, we kicked around the idea of supporting multiple categories per topic (so, more like a "tagging" approach, as opposed to categories).
In the end, we felt that having a "middle-ground" between statically defined categories and user-defined tags was not a proper compromise (since it ended up not being greater than the sum of its parts), so we stuck with static categories.
A tagging plugin can be built, of course........