Currently, Special:RandomInCategory returns a page from any namespace. It is often desirable to only return pages from a particular namespace (e.g. only articles, see T202009). Therefore, I am proposing that Special:RandomInCategory should have a second, optional dropdown for namespaces.
I created a tentative patch, currently in WIP state. Since this proposal will introduce a new condition in the WHERE clause which may not be indexed, I would like someone else to take a look at this bare-bones version and tell me if it is worth pursuing or not.
Declindes in favor of T200703 and it associated patch at https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/533728/ which will eliminate subcats form the results of RandomInCategory altogether.