One such example is collections made for certain modes that are only able to be viewed while playing that mode or selecting only one dfficulty to be added from a beatmap set into a collection
You can already add just one beatmap from a set into the collection. My apologies if you were talking about something different, but there are two buttons; one which adds the set to the collection, and one which adds only one map.
The description is a bit desert right now tho and you can already do the second thing you want by clicking only on the + button instead of the "set" button. If you want "more" settings for collection please be more accurate.
Regardless of the idea, I find the current features are more than enough...