I've started work on an 8.x branch. Current thoughts:

* Use configuration entity system, create a block_refresh config entity type, then a new entity for each block instance.
* Blocks have instance names now, so associate block_refresh settings with instances rather than the block plugin.
* use hook_block_view_alter to attach relevant js
* include a test / example block that outputs a timestamp or something so people can easily test functionality.
* unit tests!