I think Kewords are better for that purpose, and that is already possible.
Yes, we do. At least, I want descriptive descriptions and titles. The advantage of titles is, that they can be displayed below the thumbnails while browsing the photos, but the keywords not.
I like keywords too, and I am using them excessively. I am using keywords to add categories to photos - but my titles and descriptions are describing the particular situation.
I do not want thousands of keywords describing each and every event in my life. This will make the Keyword Manager impossible to use. It is bad enough that I have to assign keywords for every named person, because the names of the named people do not sync with iCloud between my Macs. Now my keyword list has already more than a hundred entries with just the names of the people. I want to reserve some space there for categories, like Birds, Lighthouses, Bears, Bridges, etc., the categories I like to use in my smart albums. Photos is now finding categories automatically, but it is not yet perfect, so I still need the manually assigned keywords.
What I mean by appending to the existing titles:
If the selected photos already have existing descriptions, and now I want to append the same text to all descriptions, it cannot be done, if the existing descriptions are different. If I just enter the text I want to add to all selected photos, it will wipe out the existing descriptions. I have to append the added text for each photo individually.