Maybe something like that
((block_id))
| Block content
| with muliple lines
block_id would be some random string bzsDAcg6B9 or ZK style 20200528210434.
To make it easier there should be shortcut/button to quickly add this block template and move there user cursor, or even modify already existing, selected text:
((autogenerated_block_id))
| <user cursor or selected text goes here>
Type (( to autocomplete, similar to Roam. Currently content can be rendered only in preview mode, later with new editor it could be visible in editor too, with collapsible header with clickable path to original location and buttons to edit/remove that rendered block. Should be obvious for app where the original is - it have to search for ((block_id with | directly under it.