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Updated: Comment #4
Part of the original plan with the changes to the theme system architecture was to model things after the structure of form API hooks. After all the discussion inwe ended up added the equivalent of hook_form_FORM_ID_alter() without adding (the equivalent of) hook_form_alter(). This breaks valid use cases.
Add the missing alter hook, hook_theme_suggestions_alter().
Add test coverage.
User interface changes
Original report by @aspilicious
While porting some parts of display suite I noticed that it is impossible to dynamically add theme_suggestions for each entity. The hooks added inrequire the HOOK name to be available in the function name.