1) if it's helpful, I can try to do an analysis of different regions that are missing data. For examples maybe somewhere actually has very few buildings, I could look to see if the Microsoft buildings open data are detected there however, and then we know that we should improve that town or region. But probably it is a rare thing.
2) For me personally, one of my favorite topics is sidewalks. Few are mapped in Switzerland I think, but many are easily visible in the aerial imagery. I would suggest to choose towns with major train stations first and prioritize mapping sidewalks there, move to a new town each month. In the long term, mapping sidewalks in every town accessible by train will help give pedestrians an idea of where to walk when exiting a train, if they need such detail.
3) a final suggestion is perhaps we have a quarterly day trip. Travel to a town, and we can focus on spending a full day exploring it and adding various details, primarily Points of Interest, especially with mobile apps like Vespucci and Every Door, but we could even consider using Mapillary, drone imagery, or just field notes to then do some detailed editing with the laptop and a Stange.