I think it would be very useful if it was possible to complete an existing library item’s missing metadata using online databases such as Google Scholar, Crossref, etc, pretty much like it’s currently possible to retrieve the ArXiv ID or the DOI of an article. For instance, this feature would make it possible to retrieve the address and publisher of a book of which you only have the author’s name and the title. Or its ISBN. Or the issue number of an article in a journal.
I know that Calibre, for instance, offers such a feature for books, and I think Zotero used to support it too for articles.
I know this feature has been suggested 4 years ago on JabRef’s GitHub, but at the time, the answer was that the Autocomplete plugin was to be implemented in the next version of JabRef. However, this never came into being because the plugin was quite old, as this GitHub issue shows.