At the moment the spec offers addons, which are components that directly target another component. Sometimes though we have suggested dependencies, something that isn't necessary but recommended, as you very well known it's done in distros. These suggested dependencies should be displayed in software centers: you want to offer cmake together with KDevelop (https://packages.debian.org/jessie/kdevelop)

Do you think it's possible to extend the appstream spec to define these dependencies?
AppStream could also use the packaging information for the final information passed to the Software Centers as well.