A lot of the work here would be aided by restructuring the forum code to create a new forum abstract class, and sub-classing it for each type of forum.

This will also give us the ability to support new forum types as a subplugin which would help to solve MDL-40385, MDL-20903, MDL-56073, MDL-3277, MDL-42981, and others.

Making this kind of change will also help to restructure some of the older code to use more efficient methods, and I hope will make it easier to rewrite the current echo/html_writer based rendering to use templates.