Sorry if this has already been covered, though I had no luck finding a similar thread.
I’m using breadcrumb navigation on a site (rsci.com) which has 5000 plus categories, all children of 9 top-tier “Main Categories”. The breadcrumb would be invaluable [in my case] if it revealed the sub-categories which existed under each crumb.
HOME >> Cat1 >> Cat2 >> Cat3 >> you are here
If cursor is placed over Cat1, Cat2 or Cat3 it would reveal a hierachy of sub-categories in each.
Something similar would have to be done for categories. You would not need the do_post_hierarchical() override, and would replace the post_parents() override with one for term_parents(). Within it you’ll need to use wp_list_categories() instead of wp_list_pages().