There really aren't any. No candidate has withdrawn or died before the election, and nobody ever really considered the possibility even as candidate ages grow older and older.

They would just Weekend at Bernie's him.

Someone could expose the whole charade before the end of the Primaries... And he'd still get the nomination.

I think all his delegates would become unbound and there could be a floor fight at the convention if someone steps up to challenge Haley.

I think the real reason that Koch foundation, etc al, funded her campaign until last week was still that they'd have somebody qualified in case it's rolled that Trump is ineligible.