I suspect if Barack Obama wins in 2012 and actually succeeds in doing what he intends to do that the Michelle option will be on the table.
She will have acquired many if not all the creds that Hillary had when she declared. It is not likely that the President's shadow would seem as onerous as Bill's shadow did to those who were worried abut ex-Presidential meddling..
She is familiar with the aims and in agreement with the style of the President and could probably be as convincing an advocate. She was in fact the President's first boss-mentor. She is his equal in every way and that is quite a statement.
I also think there are few options as attractive among the existing Democrats and that anyone we don't know would need more time to gain traction for 2016.