But will it add any new created test case to an already existing plan when the plan uses QQL and the new test case matches the filter criteria?

This can be done now with the QQL-based test runs.

Queries allow you to create a request that would look up the cases by specified conditions. The query can be then saved, and used as a source of the cases for test run - whenever you create a test run and choose the saved query to be used, it will automtically pick up all the cases matching the conditions.

Test cases should be added to the run / to a plan as soon as they match the filter criteria.

Test cases should be removed from the run as soon as the filter criteria of a run change or the test case is changed so that it no longer matches the filter criteria of the run.

The whole thing has to work via the web frontend as well as via the API.

I was about to create this exact feature request.

Indeed, on repositories where test cases are created and modified non-stop, this would be extremely useful.

Without this functionality, fixed test plans are a pain to maintain.