According to the API docs, an app can only fetch information about the logged in athlete. It'd would be nice if there could be a way of reading the following athletes' activities. The same way, having a proper getClubActivitiesById would be helpful too (right now this method returns incomplete information, which can't be used to identify neither athletes or single activities, as there are no identifiers).
Right now, one can get this information both through Strava's mobile app, and web app, either through the user feed, or just going to the followed athletes one by one and looking at their latest activities. It would be nice if this could be replicated through the API (something like getAthleteActivitiesById, or returning SummaryActivity through getClubActivitiesById, instead of ClubActivity).
These features would be really helpful for apps that aren't athlete-focused, but club-focused. For example, given an app for clubs, which is built on top of Strava and managed by a club member, this app would want to include info. about all of the members' activities. Right now, this cannot be done unless all of the members enable the club app, which won't scale as people join or leave the club. Note that by club, we can be talking about not only athletes clubs, but brands, stores, companies...