I play on PVE server so this suggestion is based on that perspective.
It would be a nice improvement, if the new Follower system would include a change to the triggering condition of when a follower will defend the player in combat.
Today, it appears that a follower will defend a player when the player actually takes damage. I can shield block for 20 minutes and not take damage and the follower will just stand there and watch until I put my shield down and allow a hit to land.
I propose instead the follower defends the player when the player gets agro from an enemy instead.