Shared folders: Navigate folders or gallery preview for photos/files
After sharing a folder (thanks for adding the ability to share a folder!), give the ability to navigate the folder tree to individual files. Then for photos or any file that can be previewed, give the option to preview the file. Think the Preview app in OSX, it works really well.
I use the ability to share a folder to give access to photos of my kids to their grandparents and family members. Switching from Sync, Sync offers the folder navigation feature I described above, but what would set Proton Drive apart even more would be a "Photo Album" viewer.
Share a folder, then when someone views the shared folder there's a button "View as Photo Album". This viewer will treat each folder as an album in the viewer. The viewer is just a different, opt-in, view when viewing a shared folder. It would show a preview of images, easy keyboard navigation (arrow keys), simple search by album/folder name, ability to download a single photo or an entire album/folder. Folders can contain folders, that's fine, it would be treated simply as a nested album.