Since there seems to be no movement anymore for C9, maybe some things can still be achieved for C8.
1.5 years ago, implementation of conditional parameters was started, maybe it can be replicated again for C8.
Using the "showon" attribute would make it easy to implement --> see here (end of the page)
Based on the information here "form->getInput" should be replaced with "form->renderField" to make it work. Maybe an update of formhelper.php could achieve it.
Active community members might be willing to submit the respective XML files to fasttrack the implementation of this feature.
Since there seems to be no movement anymore for C9, maybe some things can still be achieved for C8.
1.5 years ago, implementation of conditional parameters was started, maybe it can be replicated again for C8. Using the "showon" attribute would make it easy to implement --> see here (end of the page)
Based on the information here "form->getInput" should be replaced with "form->renderField" to make it work. Maybe an update of formhelper.php could achieve it. Active community members might be willing to submit the respective XML files to fasttrack the implementation of this feature.