Offer a simplistic Gazebo replacement which sends robot velocity commands to TF as the robot's odometry to completely remove the need for complex simulation for testing higher level behavioral attributes not related to perception or control.

This would be unable to generate sensor data, only robot motion, so in using it the documentation should mention that a map is required and have the static map in the local costmap to avoid obstacles. AMCL will probably need to be disabled in this mode and the simulator just publish the robot's map-odom pose since we know perfectly where it is from initial pose + perfect odometry.

A nice thing about this is that you can spawn the robot anywhere for some deterministic tests repeatedly

This would be a nice medium sized contribution from an outside contributor looking to make an impact!