I have always felt that the red line tool could be significantly improved by implementing some of the annotation abilities present in other Autodesk products. For instance, in regular AutoCAD you can set up an annotative text style which displays text at a reasonable size and location no matter the camera distance or angle.
I would like to see this ability in Navisworks - when you create a red line, rather than requiring it be associated with a single viewpoint, have the redline be a object in the model which can be seen while moving the view around. Additionally, it would be nice to have a few basic options, such as: being able to hide/unhide the red line, have it be "anchored" to a call out arrow, have text wrapping/text box ability, and as mentioned, have the ability to "scale" with the zoom level or always face the current view.
I believe this functionality would be especially useful for review of models, most especially after a 4D animation has been set up. A project manager should be able to drop in a red line comment which is anchored to a specific place in the model, and visible to anyone who opens the model thereafter. Similarly, have a modeler be able to create a red line pointing out object collisions which a reviewer would see easily.
As an extension of this functionality, some form of automated red line generation would be useful. For instance, be able to run a clash detection and have the results displayed not only as a list and a set of viewpoints, but as a set of persistent red lines which you can zoom in on and see where the collision is more clearly.