We can spectate some skilled mappers and learn some stuff from them. It would be better than asking them about mapping. Also it can make modding easier as well i guess.
But then you also need the .mp3/.osu, and most of the time those are not uploaded If they make small changes, it isn't interesting I don't think this possible even can get added because of all those missing files
Well this is a subfeature of that feature, right? I was always imagining that collab feature would work a lot like google docs where you can have multiple people editing the same thing. This would be same, only with no editing permissions.