The "Create Page" form gives the user lots of freedom by having an input instead of a select for the Space Name. This way users can create pages in spaces that are not defined. Users can achieve this behavior also by creating a new page from the URL.
When creating a new page into an undefined space, if the WebHome of that space doesn't exists, it should be automatically created. The content of this auto-created WebHome can be the Dashboard for that space, listing "Recent Changes" and "Existing Documents" in that space. This improves the navigation and the findability of created pages for that space.
Having an auto-created WebHome will also improve the spaces representation in Main.Spaces (in Dashboards), because users will not have to wonder why there is a space in there with a "?" icon next to it, that will attract their attention, but that doesn't "exists" and doesn't have their recently created page.
XWIKI-5374 , the non existing links (.wikicreatelink) consistency is broken, because the breadcrumb shows "WebHome >> PageName", but "WebHome" is not marked as a non-existing link.