McCaskill's husband is in the ICU for heart surgery per her twitter. I hope he gets better. If doesn't, I could see McCaskill retiring. Morbid, I know, but something to consider.
She might retire anyway, considering she's already had one medical curveball to deal with (see above). I don't think it's tremendously likely, since - and I say this with all the respect in the world for her - McCaskill is sort of the ultimate career politician, but I also wouldn't be surprised if this made her finally step out of politics.
If she does, our best candidate is obvious. Considering how close he came to unseating Blunt in the ultimate banner year for Missouri Republicans, Kander would probably beat anyone the MOGOP put up against him.