For me, the best case scenario for Federer's remaining career would be for him to launch a final push just for the two remaining majors that he has a chance of winning (USO 2013 and Wimbledon 2014), and for him to win one of them and then retire on a high a la Sampras USO 2002. It
What I really don't want him to do is to labour on for the next 3/4 years, continue to drop down in the rankings, and continue to lose habitually to mugs of the Stakhovsky ilk. His game isn't getting any prettier as he gets older, too.