If #2044969: Break the old router for contrib is a go then it is now critical to document the router. There is zero documentation as far as I am concerned, the rough overview over at Symfony is inadequate even for them and doesn't cover Drupal at all. We need flowcharts, internals (so that one can debug it), explanations (why do we use _controller / _form instead of just form? etc) and examples.
Alternatively, just roll it back it for Drupal 8 and postpone it until Drupal 9. That'd be a lot better.