(a) When viewing the photo gallery, optionally, display the photo's caption/title.

This would have to be done carefully as there are many candidate places in a photo that a "caption" of sorts is stored: jpeg caption, jpeg title, etc..

(b) The ability to edit the "caption" metadata and then modify the source file.

This means that all the effort that goes into captioning photos would not be lost as the edits would "stick" with the photo. If completed, this would make Proton Photos better than any other cloud Photo applications - as all the other store the caption away fro the photo and it gets lost when you want to later download the photo which locks you into that platform forever. So much so, that people often don't bother with captions.

(Because of the encryption at rest, This may have to be done by the client app and the modified file then re-uploaded.)