There is a categories heirarchy mod, but I don't know if it will do what was asked. I know it will give you subforums, but not sure about the "view last posts".
And yeah, the site is great like it is. There is nothing wrong with adding new things, but just remember, it takes a while to add all of the upgrades you want. I am glad that you are incorporating them yourself and didn't use one of the pre-modded boards.