President Monson is the president of the Twelve Apostles, and so probably will be the successor to President Hinkley.
Another will be called to be the Second Councilor. They may be from the Twelve Apostles, but may not be.
It might not even be a current general authority--although that is highly unlikely.
When J. Reuben Clark was called into the First Presidency he was serving as a ward gospel doctrine teacher in Salt Lake City.
But then Clark was a highly accomplished attorney and diplomat, having served as Coolidge's Undersecretary of State (where he authored the landmark Clark Memorandum which more precisely and narrowly defined the Monroe Doctrine) and ambassador to Mexico.
Faust was a good, down-to-earth, gracious, and eminently decent man. He will be missed.