If constexpr requires requires { requires } in C++(think-cell.com) Probably the two most useful features added to C++20 are requires and requires. They make it so much easier to control overload resolution, and when combined with if constexpr in C++17, they allow basic reflection-based optimizations in templates.
Template Mixins(dlang.org) A TemplateMixin takes an arbitrary set of declarations from the body of a TemplateDeclaration and inserts them into the current context.
Wikipedia's list of 1401 Infobox Templates(wikipedia.org) This is a list of all pages in the 'Infobox' pseudo-namespace, excluding subpages. Redirected and deleted templates should be removed from this list. Deprecated templates should be marked as such. 'Holding cell' next to a template's name indicates that the template is currently being deleted or merged, so no new instances should be created. Nesting indicates a wrapper infobox. Templates with over 10,000 transclusions are in boldface. Infoboxes for sport biographies are listed under sportsperson.