They cannot choose Obama or Bill. If she served two years of a term and two remained, Bill could have the VP whacked, be named VP, and then move up to her spot upon her demise. A president can serve up to 10 years, only two terms of which can be by being elected.
But he could become Acting President. The Speaker of the House is next in line after the VP. If the Democrats had control of the House they could select him as Speaker and if both the Presidency and Vice Presidency became vacant, he would become Acting President.