We never got Persona past the point of needing an external script on websites pointing to the Persona domain. We really don't want to risk anyone getting XSS'd or assume transitive trust in whomever would follow Mozilla in custodianship of the domain, so we plan to kill the service, destroy all user data, and maintain ownership of the domain for many years.
Also, Persona was pretty explicitly designed in a way that assumed eventual, native integration into browsers. IMHO, any successor without the backing of a browser vendor would be better served by starting from scratch with a different set of assumptions. :)
What about a browser plugin? Perhaps support could be added through browser plugin, without explicit support of browser vendors. Then when adoption is significant enough, perhaps browser vendors will get on board.
(Hypothetical future in which Persona takes off. I think it's a great idea.)
Also, Persona was pretty explicitly designed in a way that assumed eventual, native integration into browsers. IMHO, any successor without the backing of a browser vendor would be better served by starting from scratch with a different set of assumptions. :)