(I'm sorry if this task does not fit but I've just started using Phabricator.)
I'm seeing the feature Page preview more and more often. As I use Wikipedia for math and physics, too, I'm often interested in having, e.g. a definition in the page preview. That's why I thought it would be a nice feature if it was possible to define on a Wikipedia-page what text should come into the page preview. Maybe this can be automated by looking at different categories (physics, philosophy, ...) and looking at what there is generally wanted in the preview.