I think it would be nice to have a class (character) forum for each character so a discussion could be had on each.
This would make finding the current theory crafts easier along with facilitating discussions on spec improvements. As an example the WoW forums have a section for each class, and within them there are stickied & community updated threads with general knowledge about classes to help out new players or people interested in trying a different class/spec.
I don’t think individual characters or Careers can spark enough different topics to justify an additional forum category. If you want to find more people to talk about specific builds, optimisation and such, try the Lounge here, or turn towards Reddit (there’s a lot more build-related stuff there already) or the various Discord groups (including the officcial one).