This is also something I really like on goodreads, that I feel is missing from MDL. On goodreads, you can create a series out of books, for example, for books that are from a trilogy, a box-set or a universe that an author created. You can create a numbered or unnumbered list.
For MDL, this function could be very useful to link "Drama Special" and "Drama Stage" specials together. Until now, you have to search for them and scroll through the results, or go to some other site like wikipedia if you are searching for their numbering. It could also be used to link and number an anthology/omnibus, for example the three movies that make up the "Three Colors of Fantasies".