In #26355 we've introduced an initial version for the template editor mode in the post editor. This highlighted a few issues in the design/flows of that feature. The current issue is an overview tracking the progress for all oft these follow-ups:

Blank Canvas Template

• Allow creating custom block templates in classic themes Allow creating custom block templates in classic themes #30438
• Better distinguish between the content editor and the template editor. The users should be able to quickly tell which mode they are currently working with. Site Editing: It is not clear at-a-glance whether one is editing a template or an item of content #27849

Interface

• Allow deleting new custom block templates, and reverting customized block templates in the template editor It should be possible to delete new custom block templates, and revert customised block templates in the template editor #31076
• View the template while editing the post Post editor: Provide a way to view the template while editing a post #27847
• Design UX patterns for blocks based on context (content editing vs template editing) that enable users to flow between content editing and template editing directly on the canvas. Site Editing: Restrict block editing capabilities based on editing context #27848
• Site Editing: Explore a pattern for template part editing that is consistent in both content and template editing contexts #27852
• Allow selecting custom page templates. Add support for custom templates in FSE themes #27778
• Find a visual way to list the available page templates (actually preview the templates and maybe show them in a starting modal). Site Editing: Visual template selection #27850, Refine template flows in the post editor #31591
• Give the user the possibility to save the changes to a template specific to the current post or a custom template that can be used in other posts too. Site Editing: Allow template edits to be saved as a new template #27851
• Add a welcome guide for the template mode. Site / Template editor welcome guide #29031
• Colorise template parts and reusable blocks in the UI Colorise template parts and reusable blocks in the UI #32163
• Mosaic View of Theme Templates. Mosaic View of Theme Templates #20477
• Bug: Template part click-through Click-through pattern not behaving as expected for Template Parts #35079
• Refine the Site Editor loading state #35503

Infrastructure

• Allow using post meta values as "sources" for some of the blocks used in the template.
• Allow themes to opt-in/out of the "Create new template" flow.
• Support dynamic values in block-based templates. Block Based Themes: Dynamic values in static HTML theme files #20966 (Try) Template Loader: Allow PHP in block templates and parts #25316