This has a lot to do with current events, but really the POD would take place in 2008 shortly after the US presidential election, in which Romney decides that it is unlikely he has what it takes to be elected as President of the United States. He still raises campaign funds for fellow Republicans, he makes speeches and appears in the media, but he informs all of his supporters that he won't be gunning for the Republican nomination in 2012.
What this could mean is that he decides to run for Senator in Massachusetts in the special 2010 election... I'm thinking he would probably win. He still publishes a book. But he stays out of the 2012 election. What butterflies could we see by now? Who would be at the forefront of the race?