Hillary won't appoint more than one Senator to the cabinet. The pros of having McCaskill as Secretary of Department of Whatever surely aren't worth the cons of losing her seat.
I can't see Hillary appointing a lot of incumbents to the cabinet. She's too smart to do that, and she has a massive network of qualified supporters and yes-men within the federal bureaucracy.
I'm going on the assumption a Dem governor appoints a Dem senator to serve until 2018 anyway, and McCaskill wants to get out of the Senate rather than risk a loss. In that scenario, no net loss in the Senate and Clinton gains a strong female cabinet secretary.
If Koster loses or Missouri senate vacancies don't work that way, forget it.
