Right now we can make an activity 'private', 'followers only' or 'everyone'.
I'd like to see more granular permissions for the "followers" category, where we can further break that down into groups we create - and even have a group where we can add users we want to exclude.
Basically, the same way Facebook lets you decide who can see your posts, so for example, be able to do something like "All followers except those on the exclude list"
There are many reasons why someone might want to do this, perhaps they are training for something very competitive and don't want ALL their workouts shown to their followers. Or perhaps they are working out on their lunch break but it could get misunderstood that they are skipping work for a workout and don't want their work colleagues seeing those workouts, or a myriad of other reasons.
This is different from outright blocking a follower. It would only hide certain activities from those excluded or not part of the default groups you create.