One case for not using Wikidata for Geneology would be that specific subtrees should be nonpublic. The present system prototype that builds on MediaWiki won't be able to answer that use-case.
Another case would be the desire to allow people to upload data that's not based on any sources. If that's the desire for the new project, then there's a completely unanswered question about quality control.
So, focus on coding something independent of Wikidata?
Very good point about some subtrees needing to be private. The usual solution to this with MediaWiki is to have separate wikis for each group of users. This approach could work well with genealogy wikis too, I think — although, not, as you say, if the data is all on Wikidata.
As for quality-control, I think we should be aiming at a system that's easy for people to install themselves, so their unsourced content can be hosted away from the main central communal wiki.
The current prototype doesn't use Wikidata, and is mostly just focused on building the tree/graph of person-pages (and doesn't impose the family structure of Gedcom).
I may be wrong here, but doesnt different SQLs within Wikidata give a very useful and good way of keeping spcified records nonpublic. if this has been configured?
No, I don't think there's any way within Wikidata to make any records private. Perhaps there is in Wikibase (the extension).
A project to do this, which will hopefully eventually link with Wikidata, is History Research Environment (HRE): https://historyresearchenvironment.org. Developer support is most welcome!
This is actually a great idea 💡, it could also have a "Find a grave 🪦" feature that utilises Wikimedia Commons photographs of gravestones which could be connected through Wikidata. Such a sister project would have millions of possible uses, how can I support the creation of such a project?
Familypedia uses "Instant Commons" and can incorporate those photos with
ease. WikiTree can probably use them easily too.
Robin
https://familypedia.fandom.com/wiki/User:Robin_Patterson [1]
There have been various discussions over the years about using InstantCommons on WikiTree, but it's never been enabled, because (I think) they prefer to keep everything within the one site. I suspect their version of MediaWiki is so old and modified from the mainline code that it may not even be possible any more.
Also relevant to this task: there has been recent discussion about closing the proposal for a Wikimedia genealogy project.
I'm not quite sure how to resolve this task, as it seems more like a discussion and isn't proposing anything in particular. The current state of non-public genealogy data in MediaWiki I think is to set up separate wikis where different access control is required.
There is discussion on-going at the moment about a new proposal called Wikifamily, and general discussion is at https://meta.wikimedia.org/wiki/Talk:Wikimedia_genealogy_project so let's move this there.