After the battle of Chancellorsville, General Sickles was recovering in New York City. As Sickles stated, “I would not have seen Gettysburg had Hooker not sent me a message summoning me from New York, where I was slowly recovering from a contusion received at Chancellorsville”. But what if instead, Governor Seymour asked Sickles to command the New York militia that was being sent to the Department of the Susquehanna and General Couch. It would give him the opportunity of independent command leading his New York troops and possibly other serving emergency militia. As a Major General with rank from November 29, 1862 he would be senior to all but Department Head General Couch. This assumes that more troops arrive at Harrisburg from both New York, New Jersey, and Pennsylvania. This would allow General Sickles to follow up General Ewell’s march south from Harrisburg to Gettysburg. While the militia is no match for the Confederate 2nd Corps a delay by as much as a day could change the initial stages of the battle of Gettysburg.
If the militia troops were held in reserve during the battle they could be used to follow up the Confederates retreating to the Potomac. Troops near Chambersburg (close to there supply lines back at Harrisburg, could force Lee to travel to the river crossing through Waynesboro instead of New Gilford.
The other part is what happens to the Third Corps in Sickles absence. His two divisions could be used to bring other Corps up to strength. While Sickles may protest afterwards, there was nothing he could do to stop it.
These are just my random thoughts. What do you think?