Oprah Winfrey. Or the closest he can get to Oprah.
Think about it: every personal fear voters have about Trump will be assuaged by having her (or someone like her) on the ticket:
Too volatile? He'll have Oprah there to calm and counsel him.
Racist? How could he be, if Oprah is running with him?
Anti-woman? See previous answer.
Remember, Trump has approached the whole campaign like a reality TV show. He's not going to ask, "Who would be most qualified if I die?" (not that conventional candidates do, but they pretend to) but "Who can I be paired with that will give me the biggest boost in ratings?"