This isn't exactly an extension but another Strava-connected service.
https://wandrer.earth
Just ran across this last night - unfortunately everyone else did too I guess and it started to stress their bandwidth with Strava. Intermittently down last night but up today with a warning message, and slow on processing rides; understandably.
It shows you where you have ridden and places you haven't ridden, percentage of the Earth you have ridden, etc. Not sure if I explained that properly... Leaderboards, points, etc.
On a side note...It's interesting because a casual goal I have had for a few years is to ride every (public, paved) road in my county just for fun. I don't make too much progress with it but I try. I know others sometimes do something similar, especially for cities.
So one day while my wife and I were driving somewhere I outlined a plan and the process in detail of how I would set up a service that would allow a person to set a parameter or area that they wanted to ride and then connect the service to Strava to show how much of it they have covered, how much they need to and map out the areas they are missing.
Big complicated project, too huge for me and likely too technically advanced, some money involved, dedicated server in case of traffic increase, etc etc etc.
This isn't exactly what I envisioned but it's leaning in that direction, which I thought was interesting.
For now I use Strava Summit Heat Maps and Jonathan OKeefe's Strava Heat Mapper -
https://www.jonathanokeeffe.com/strava/map.php