A feature to compare the output of two nodes, both in 2d and 3d viewports. Here are some suggestions on how to implement this:
1) Nuke style
The viewer node has multiple inputs, two of which can be selected on the left and right of the viewport. The viewport has controls for blending A and B, or for wiping between A and B using a moveable cursor.
2) After Effects style
Screenshots of the current viewport can be stored and instantly accessed using UI buttons and keyboard shortcuts. Not as good as Nuke's option, but faster and still very useful.
Doing this in 3d might be complicated, but just having this on 2d viewports only would already be great.