The basic FirstItemId identifier strategy we provide is not very helpful in interfederation use cases, where the ItemId really needs to be supplemented by information about the entity's registrar to be of use.

I have been using a registrar-sensitive identifier strategy in the UK federation tooling for some years now; although it can't be used as-is (it also handles some special cases for UKf entities based on the ID attribute) something with the rest of those capabilities would be useful for a lot of people.