He intended to groom Anthony as his successor. We see that in the bedroom scene after the Tahoe shooting, and at Michael's party in the beginning of III. When the chips were down, he trusted Tom to run the family while he was away, temporarily.

BUT: No, he did not have a succession plan in place in case of death ("natural or otherwise") before then. As you said, Vito could rely on Sonny to run the family, and Sonny did after Vito was incapacitated. But, if Michael had been killed in the shooting, or didn't make it back from Havana, there would have been a (probably) bloody struggle for succession.