A crowdsourced map with a collection of "favorite routes" would be an interesting idea, although some people (including me) hate trodding down beaten paths. I suppose these are mostly on websites/specialized apps, eg. for hikers.
Actually you can fit your needs with Openstreetmap. People regularly upload public GPS traces that you can view - thus, get a rough idea of how popular a path is. But OSM also shows all the paths, so you can avoid beaten paths as well.
Strava has a global heat-map for running, cycling etc. although I believe it is a subscription-only feature. There is also your personal heat-map which is cool to see if you use the app enough.
Not subscription-only, but „ Street-level detail is available only to registered Strava athletes“ on https://www.strava.com/heatmap so you need an account.