Allowing seperate category trees for different URLs would be very useful for a site with multiple unrelated subsites. I don't think it is possible to support this with the current structure it is very simple. a table of keywords and a keyword map table that maps each keyword/content-item pair.
Also allowing package-based category trees would be good. Bboard and other packages now implement their own keyword lists and maps. The keyword system needs to be standardized across the system if it is to be useful.
Category based searching also should be on the list, but we need to get the packages using the keywords before that will be necessary.
There is definitely alot of work that can be done to enhance the keyword system. I had just been thinking it might be advantageous (or maybe not) to allow keyword assignment to acs-objects instead of just content-items. That depends on whether your opinion is that all content for the entire site goes into the content repository.
