The 80-400 was one lens that was severely in need of an update and when the new one arrived it delivered what was needed other than gaining weight,
if and
when a new 300 F/4 appears I think it could reasonably expected to deliver significant advances on the "ancient" specifications it has and they would probably include a weight reduction due to newer composite materials along with improved optical quality and "modern day" VR.
At the moment it is all pure speculation about how a new version would be but I reckon in all certainty the price will increase substantially to "modern day" levels as well.