Longstreet would have been the temporary successor, as happened when Lee was unfit for duty in the early stages of the maryland campaign. The main choice would probably have been Longstreet. For the reasons being that
A. Longstreet was his undisputed second and was going to take command of the ANV when Lee was possibly going to go west to Chickamauga.
B. Beauregard, Johnston and others were loathed by Davis, so probably would be promoted and was viewed by the ANV as inaffectual commanders And Kirby Smith and the like were too much of outsiders.
C. If they couldn't find a replacement for Bragg after his subordinates complained in 1863, why would the create that needless gap if Lee was killed?
D. One of Lee's other commanders simply did not have the talent. Jackson was not a good adminastrator, Early was not well like and fair at best, Ewell was indescive, Hill had no initiative and was ussualy ill, Stuart was to young rash and not suited for infantry, Anderso was merely adaquete and Gordon and Hampton were non proffesionals. Thus, I believe Longstreet would have been the pick.
“Duty is ours, Consequences are God’s.”
-Lieutenant General Thomas Jonathan Jackson