This thing is so much fun! I love wandering around a new area and filling in the details.
One thing I wish it had was the ability to add an entirely new building. I'm currently living in a town that is experiencing massive growth and I'll often see a structure that's still a vacant lot on osm, and I don't think you can add it with StreetComplete.
The easiest thing to do for such cases is to add a note, perhaps with a picture of the building if you're comfortable.
There must be folk who dedicate their time to searching the OSM database for unresolved notes, because whenever I leave a note on StreetComplete mentioning an inaccuracy (e.g: a path where there isn't actually one, a driveway marked incorrectly as a road, etc.) I get an email alert a few days later notifying me that the note has been resolved - often by someone who lives in an entirely different country (and who can use the information I provide in the note plus the satellite imagery to resolve the note).
One thing I wish it had was the ability to add an entirely new building. I'm currently living in a town that is experiencing massive growth and I'll often see a structure that's still a vacant lot on osm, and I don't think you can add it with StreetComplete.