Stephen Hackett:

This solution is what I would prefer. In it, my wife and I would continue to have our own libraries, tied to our own iCloud accounts, just like we do today.

However, we would be able to add each other as viewers to our libraries. Once access has been granted, her library would show up in the top-level of Photos.app as do things like People, Places, Videos, etc. I could tap her library to see all of her content, including albums, collections, etc. If I were to see something I wanted to add to my library, I could select it and tap “Add to My Library.” Doing so would prompt iCloud to add a full-resolution copy of that file to my personal library.

The one thing keeping my wife and I from using iCloud Photo Libraries is the lack of a feature like this. Right now, we use Google Photos with a shared Google account on both of our iPhones. This gives us a single, unified library to browse and offsite backups for our photos. If Apple shipped a library sharing feature, though, we’d switch in a heartbeat.