For example, I've completed my run 15 km, but I also want to know how many of 15 km were of the track I'd never run before. So for every activity it would be the additional parameter - 'new miles/kilometers'.
As a developer I realize, technically it's not easy to implement, some advanced algorithms should be used, but it will pay for itself. It could be such a tidbit for many users and can encourage them to subscribe!
That feature would target a pretty big amount of users who travel a lot. They would love to know how many unique kilometres they have run/ridden. And to compete with each other on this.
(And, yes, some third-party developers have already created it outside Strava.)