I think it will be great if Strava adds Time Trial Challenges.
What is it?
The time trial challenges are a set of challenges based on time and not distance covered. The athlete has to cover as many miles as possible (AMMAP) in 30 minutes, 60 minutes, 90 minutes, etc. depending on their sport.
Why add time trials?
This challenge literally puts the athlete under the clock to test their lactate threshold (times may need to be adjusted based on sport).
How does it work?
Time trials can be an activity (run, ride, swim, row, walk, etc.?) that the athlete marks as a race. Another option is to allow the athlete to start a time trial from the challenge screen directly and see the clock counting down. Time trials allow athletes to select which activity they want to do a time trial for in that month. Then the athletes can try to beat their own time trial PRs month over month.