Clearly by looking at the items above missing test cases, IMHO many test cases need to be created so that others have examples of how to use the predicates. I know in initially learning Prolog not having examples was what kept me from using those predicates.
When I note that the test cases should be created, that means we the SWI-Prolog community should contribute to SWI-Prolog by writing the test cases and not expect this of others.
Also in creating the above list it needs to be further segregated out into different data structures such as list, rbtrees, association lists, facts, dicts, nb_set, ordsets, tables, etc. as many predicates for one data structure do not operate on other data structures. However many of the libraries tied to a data structure come with predicates can translate to/from Prolog list so all is not lost.