Hillary stumbles at the last moment, maybe after a major debate. Michelle steps in, takes the nomination.
(Sorry, no firm prediction as to if she'll win the general election. But I don't think she could beat Trump.)
The Obamas are vain enough to think she could win. But I can't believe she would. She has none of Barack's glibness and superficial charm. Her anger is clear for all to see.
On the other hand, her ideology is purer than Barack's. He's (in approximate order) Muslim, Marxist, Racist, Green, and Queer. She's racist, racist, racist, racist. It could make a difference with the squishy white self-hating liberals if it's so obvious.