Gradient Adjusted Distance
Many training plans provide guidance in the form of distance to be run in each session, but these distances are all based on running on the flat. In the same way that Gradient Adjusted Pace normalises pace for different elevation profiles, it would be really useful to have the option to view a "Gradient Adjusted Distance" in many of the places where Distance is presented.
This is not the same thing as 3D distance, which makes relatively small differences except in extreme terrain, but rather a metric which converts the effort from climbs into an equivalent in terms of miles.
For example, on one of my recent easy runs, I covered 10.28 miles at 9:40/mi, 8:59/mi GAP, with a moving time of 1:39:28. If we applied the GAP pace to the time run (1h39:28 divided by 8:59), we get a "GAD" of 11.07 miles, which is a fairly significant difference (and the hills weren't even that extreme on this run). This metric is one which could be calculated very simply from existing data fields (Time / GAP), but could be a really useful concept for tracking training volume.