I’d like the ability to use PhotoLab on a laptop to process some images — pick, rate, keyword, and then process with the full range of tools — and then later be able to move those photos to my desktop computer such that PhotoLab there will see all of the work I have already done on the laptop and allow me to continue processing/roll back as required.
Doing this on a per-photo basis would be fine, quite functional, but the ability to create “libraries” and import/export/merge them would be extra useful. I see no need to sync between libraries (personally).
The one area of complexity I see would be the keyword database, as this exists independently of the photos. I guess it would make sense to be able to export and import this list and ideally merge them. It would be possible to manage without a merge (all merges are tricky) with some care.