My response, also posted there :
I'm a great believer in LocalIndexes, basically pages on wiki which are just lists of other pages about a particular theme. These seem to me to play the right role in MiddleSpace between one big, TopDown taxonomy, and the completely unarchitected "floor". Local indexes are not the same as CategoryCategory (which is completely bottom up) : someone deliberately goes round and decides that these ten pages need to be clustered within a theme, and makes and maintains a page for that theme. (eg. something like OnCities. At the same time, the local index page doesn't itself need to be part of any particular higher level structure. Nor is there a problem with cross-linking between two of these indexes (with each as a list-item of the other) so you don't force hierarchy.
Knowledge of such local indexes can be "lore". But it's also easy for newbies to learn. At the same time, there's no harm if other people who don't understand the structuration build their own alternative. They can quickly be connected with a "ReadWith" or manually merged to the other.
See also :