Hello,
I don’t know all the things you are talking about, but as a user with a little bit of technical skill I ask myself if it would be a good idea to make step by step.
- There are standard EXIF Metadata like is described here http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf. This standard data DPL must be able to read/write, and for working with the search function of Photo library DPL must be able to search with search criterias (and/or/not…) my photo collection. If this works well I will be happy.
- Photo Metadata User Guide (iptc.org) would be the basic for the next step working with metadata/keywords writing to XMP files.
- Database design and function I’m not interested…for me as user it has to work
. I must have the possibility to backup and restore (it’s implemented), by closing DPL it would be fine if i got a message to backup ( like LR does it with the catalog) - I would be glad if DPL could separate program data, user data and user config, and if I would be able to choose the location where user data and user config will be saved. At the moment I can only choose this for database and cache. Presets, Plugins … are anywhere in the program folder and/or in the user profile (Windows system…don’t know where in Mac OS). So for me it’s not so easy to backup these files if I made custom presets, palettes, workspaces. Watching forum threads I noticed that also for users working a long time with DPL it’s not easy to remember where all the stuff is stored.
If anybody is here knowing about terminalServer/citrix environments some years ago, she/he knows how difficult it is to bring a software not designed for multiuser environment to run (this only for a synonym seperating data)
That’s the sight of someone who likes to take photos, want to keep his data safe and easy and not interested to crawl or dig for parts of the data.
So keep on discussing, I will go outside because here the sun is shining and the air is clean.
Please keep all in good mood, stay healthy and enjoy the coming christmas days 
with your family
best regards
Guenter