Problem/Motivation
If a theme hasn't provided base theme setting, the theme will currently fallback to extending stable. This works when there's only one version of stable. However, in Drupal 9, there will be multiple stable versions (Drupal 8 stable and Drupal 9 stable). This will either be so that Stable is moved to contrib, and a new renamed Stable theme takes place in core, or so that there will be multiple versions of stable in core. Both of these scenarios need themes to explicitly define which version of stable it wants to extend.
Proposed resolution
Deprecate the option to leave base theme empty. Provide warning for themes that haven't configured their base theme and provide them with instructions that if they want their theme behavior remain the same in Drupal 9, they would have to set the base theme value to stable. In Drupal 9 base theme will be a required variable in theme info.yml.