Please, not Castro.
In the twenty first century, the Vice President has to be someone who can take over the Presidency and be able to carry out the duties of President competently and credibly.
Castro is a light weight, and is simply not prepared to become President.
The Vice Presidency in this day and age is simply too important to be determined by tokenism.
I basicly agree. But I think we won't have a succeeding Vice President (through acension under the 25th amendment, not elected like Poppy Bush) for the next time. The last one was in 1974. The chances for a president to die in office are much lower than they were fifty plus years ago.