I think it would be cool if Strava showed a live stat for how many athletes are active at any given moment.
What is it?
An athlete can go to a page and see how many people around the world are recording an activity right now (separate stats for running, cycling, swimming, walking, working out, or playing any sport).
Why is it important?
Inspiration. When we see that there are many people around the world working out right at this moment, it gives us inspiration to also go work out. It also gives us confidence that we can do it too (if so many people are doing it right now, we can too). And it's cool to see the number of athletes working out around the world.
How does it work?
I think it could live under challenges so that anyone can see how many people are working out right now. Another option is to put it as a link athletes can access from the Strava website that doesn't bother the user experience. It could be divided by country, state, or city if the athlete wants to go into a detailed view.