Feature summary (what you would like to be able to do and where):

When pressing the menu button when reading a Wikipedia article (the button in the top right), show a dropdown context menu item 'In other projects'. When tapping on it, it shows other Wikimedia projects for selection in this order: Commons, Wikidata, Wikivoyage, other ones.

One could also allow the user to rearrange or unselect particular projects from showing there similar to how the dropdown itself can be customized (maybe on that customization page show a customize icon button for that context menu item to configure it).

When tapping on Commons for example, it would take the user to the specified Commons category. This can be very useful because there's often lots of useful or interesting media there and the article doesn't have all that much. Note that for Commons, if the Commons app is installed the user would be prompted whether to open it in the mobile browser or the Commons app (so far the latter doesn't play audio & video files but it has better UI for categories & images).

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

This is mainly about Commons (I think the main use cases lie there). An alternative idea would be to only show Commons but for some users in some cases the other projects' pages may also be useful on mobile. With Wikidata one could for example quickly find the subject's page in some specific other site and one may want to be able to quickly open a city's Wikivoyage item and so on.

Often the Commons category is not even linked in the Wikipedia article (it's often linked in the External links section at the bottom or in the See also section and in either case in not a very clear self-explanatory visible way). This may also be a somewhat-low hanging fruit regarding what the Image browsing effort is about.

Benefits (why should this be implemented?):

The media files can be very interesting. Many things are better in media files. For example statistical charts about the subject. The article may only have a few but the Commons cat has many more. But there's lots of reasons. People may want to see more photos about/of a subject or whatever else it is. Don't underestimate the importance of visual and audio content. Currently it's often not only cumbersome and varying and unknown how to go from article to the place with media about the subject, but even not possible at all from within the app. One would have to open the article (via 'Share' btw which is not clear to the user as the way to open the article in the browser) then open the desktop view for the users who are active-enough contributors themselves to know how to get to the Commons category and then in that too large page zoom in on and click on Tools to open the Commons category. That's just not realistic; essentially nobody is or will do that.

In the future maybe a panel of media from the Commons category could be shown below the article with a button 'More' to open the Commons category or even to load more directly within the app in a then enlarged panel but that's a different topic.