From what (admittedly little) I've read about the ACW, it's said that Longstreet had a better grasp of the changes in warfare since the time on Napoleon than Lee possessed.
So, for whatever reason, Lee is not put in charge of the ANV. Maybe his heart condition convinces him to retire, maybe he isn't able to shake the "Granny Lee" stigma, whatever. What would it take for Longstreet to be put in command of the Army of Northern Virginia and what would happen?