Breaking and architectural. The plugin conversion of the Legacy hook classes, plus the design-decision features that need it:

• #3069511 Replace Existing Files (33 followers, most-watched in the queue; needs a maintainer design decision)
• #2879540 generate name in presave so other modules can react
• #2762615 temp-file autoremove (Major)
• #3583291 schema_form consideration rides the conversion