It would help WikiProject members to have an easy way to monitor changes on all of the articles in their project, including both the main and talk namespaces.
Right now, you can use Special:RecentChangesLinked/Category:Foo to get RC for pages marked with a category, but this only shows results in the namespace where the category is linked. Since WikiProjects tag an article's talk page, the only items that show up in RCLinked are talk pages.
The new page will be called Special:RecentChangesCategory.
For now, it won't be discoverable (no permanent links to it on a page), but if it's useful, we'll see what we can do with it. It might be something to add as a link on category pages.