FHMWG have proposed this:
Thank you very much for this link.
I had reviewed this site some time ago, but have not had a chance to revisit.
At the time I had added a category to my utility for specifically this purpose, but at the time IIRC, there was only a single data element CircaDateCreated
Also I noted that the CPP code proposed at that time has not been changed and depends on the Adobe XMP SDK.
For me personally that SDK has been problematic (as recently as last week) when compiled under MSVC, specifically with UTF-8 encode strings with special characters such as Umlauts.
As always, I will have to look into this a lot more and appreciate both your feedback as well as the work of FHMWG
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For my use some of the most interesting tags are
Person in Image
Keywords
Location data why, where and when the image was taken, rather than when it was scanned
of course all of the data recommended by FHMWG, as mentioned by PLegoux.
To be honest, which data to stash where is one one of the things which I have not been able to nail down and is THE reason why I have raised the issue here, though the FHMWG recommendations are a very good start.
The big advantage is that the data stays with the image - unless , of course, you end up processing the file with some of those nasty tool out there which blithely delete or mangle what ever they were not specifically tested for.
If you are unsure of why or how metadata can be and is useful, maybe fire up your favorite search engine and have look for genealogy & metadata.
FWIW, Gramps is not the only app where the metadata would/will be useful.
In my case I have so many images and other files scattered over several PC and numerous backup media.
As Digital Asset Management System which can read that is the most useful app when it comes time to try and track down where that picture you are looking for is stashed away.
Or which of the dozen or more copies of the specific file is the most up-to-date with regards to the metadata.
And this is the main reason why I very much prefer a separate specialized app to edit and review that metadata.
Many of my family pictures need to identified once they are scanned so that any family member who comes across an image won’t sit there wondering just who is that person, where and when did the event take place.
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FWIW, I have looked through this file and right now the number of tags covered seems a bit limited. My attempts to see where some the data might end up are stymied right now.
Because, even though I have been able to compile and link the writer app under Win 10, it does seem to have trouble writing the sample data from the jason file included in stage1.md.
Unfortunately, I have not been able to find any place where this code is (actively) supported. I have left some queries on the fhmwg Github page, but those pages do not seem to be monitored very often and right now I am stuck.
My apologies for this rant, but this seems the only place where any one is likely to read and respond to this problem 
Vent as much as needed. I posted a similar message to the Facebook group … asking if the group’s name change meant someone might start responding to messages. (As you might find predictable, there has been no reply.)