Weatherly was on one of the Hollywood shows last week and indicated there was death involved. So, my leading theory is that DiNozzo senior dies suddenly, leaves Tony a surprise fortune, and Tony decides to turn in his badge to go find Ziva.
However, almost all of the longer enduring characters that have exited the show have died (Kate, Jenny, Mike Franks, Eli David, Jackie Vance, Dorneget, Tom Morrow). And Fornell has been shot, but we don't know his fate yet.
Thoughts?