The “Activity Pace” page says we should provide some feedback for what we’d like to see on the page, so here goes … 
I’d like to see the relationship between pace and heart rate modeled. This is similar to the relations between cycling power and heart rate in terms of what it can tell you about your fitness.
Here’s a TrainingPeaks blog post that describes how it’s used. For my runs, this is the only remaining TP feature I still use.