It could make sense to have them integrated with the Wikibase registry, so that they are available to the entire ecosystem rather than just one instance.
We don't have a plan for this yet.
Perhaps a different namespace?
It could make sense to have them integrated with the Wikibase registry
We probably need some sort of slightly automated discovery? but probably don't need to list them on the registry.
If you know where wikibase is you should be able to get the shex.
So, the current extension is configurable, using the wgEntitySchemaShExSimpleUrl setting.
In WMF production for wikidata this is set to:
For a custom install this can be configured to point at the hosted shex-simple app on tools with a custom Endpoint, or even to a custom install of the shex-simple tool.