In the issue about adding more settings to the breadcrumbs block I kept thinking that there are some block settings that you really want to apply to all instances of that block, not just one. Making changes to the header block, for example, can be tedious if you need to repeat the exact same change on every instance of the header block.
Custom blocks can store their configuration either in an individual config file, or as part of a layout. I would like propose that we provide that same functionality to all blocks.
Each block type can define whether - by default - it's configuration should be global (like header & breadcrumb blocks) specific (like the page title block) or provide an option to the user (like custom blocks). It not set, the block config should be treated as specific - so existing sites will not be affected.