It is not uncommon to have a node provide a topic publisher or subscription that could be have different valid QoS settings depending on the application. In these situations, we see node authors offering ways to configure the QoS (typically in the form of a ROS parameter). For example,

ros-perception/image_pipeline#521
ros-drivers/ros2_ouster_drivers#26
ros-simulation/gazebo_ros_pkgs#1079

I think it would be valuable to have a common mechanism for configuring QoS at runtime. Having a consistent interface for node authors and integrators to use would hopefully make their lives easier.

See ros-simulation/gazebo_ros_pkgs#1079 for some early discussion.

(edit by @ivanpauno) Early discussion in ros2/rclcpp#239.