After the disappointment with Gluu (e.g. their use of a for pay middle layer), there hasn’t been much progress on implementing an OpenID Connect auth client in Nextcloud it seems.
However there seems to be some recent work on something like that for Owncloud, see:
Might be worth a look if it can be ported easily?
Contrary to the User_saml app it also seems to support additional user attributes like groups etc.
Or am I misunderstanding the functionality of it?