For some kinds of pages, a great many kinds of slots may be allowed, and only one or two may be required, but a handful should be offered to the use when editing or creating a page.
Example: Not all template pages need to have template documentation, but it would nice if the UI would offer an easy way to create that. A generic mechanism by which a SlotRoleHandler that a certain slot should be offered when editing a given page.
Proposal: Define SlotRoleHandler::getDesiredRoles( LinkTarget ), which returns the slot roles that should be offered when editing. Note that this does not always mean that they can be edited atomically with the rest of the content, or that they can be edited using the standard EditPage at all. It just means that editing them should be offered to the user somehow.