The prevalent linking from Wikipedia (article) pages to Commons (category) pages of identic item is through "Commonscat" template, exported to Wikidata as the property P373. However, this linking is available at the Wikipedia page only using a template with a special inclusion function, and is utterly not available nor detectable from the target Commons category page.
As long as there is not possible to connect both article pages and category pages of identic item to one item page and we have two item pages for one item (first page for articles and second page for categories of identic item), we depend on linking through properties, However, such a linking is not so functional as standard interproject links. Some improvements to compensation of that deficit should be developed.
- At Wikipedia pages, the P373 link from the attached Wikidata item page should be displayed as an interproject link even if the page doesn't contain a template with inclusion function. (This point shoud be very easily realizable IMHO.)
- At the Commons page which is a target page of P373 from one or more Wikidata items, links to the item page(s) and its/their interwikis should be displayed as interproject links (in any appropriate form, e.g. as a drop-down list).
- At the Wikidata, item page, P373 should be checked that the target page really exists. When the target page is deleted, moved or hard- or soft- redirected, P373 link should be automatically updated.
- If the attached Wikidata item page links to P910 or P301 or is linked from another item page by P910 or P301, the P373 property and other interproject links should be read from both item pages (because they both are related to identic item).
Some of these improvements should be applied also for other properties which links to Wikimedia pages, such links should be a special datatype "Wikimedia page" instead of "string" datatype and they should work similarly as interproject links.