In OTL the situation in Virginia was thus: the 120,000 man Army of the Potomac sat astride the swollen Chickahominey river, effectively divided into two, while the near 60,000 man Army of Northern Virginia was camped just outside Richmond.
There were reports of Irvin McDowell approaching south with 40,000 more men and because of this Johnston decided to strike at the stronger Federal force on the north side of the river but news of McDowell being redeployed to the Valley changed this.
Johnston plan was simple.
Benjamin Huger's 5,000 man division would advance along the Charles City Road until it met up with Robert Rodes Brigade near the Federal left wing. Huger's division would wait there and succor the Confederate Right Flank.
Then Rodes Brigade would join up with D.H. Hill's reinforced 11,000 man division on the Williamsburg Road where Hill would then lead an attack on the Federal positions at Seven Pines.
While this was happening James Longstreet would lead the real main assualt with approximately eleven bridges of about 13,800 men along the Nine Mile Road to Old Tavern where the road intersected to New Bridge Road. There Longstreet would wait until Hill's division had engaged then he was to launch an attack into the Right Flank of Keyes dvision which would be weakened.
Huger understood the plan, Hill understood the plan, but Longstreet didn't. Johnston's written orders were not all that clear as he believed in allowing his subordinates freedom to take the initiative but his orders to Longstreet are unknown in its clearness. Johnston spen seven hours with Longstreet discribing his plan to him, as Longstreet had the most important job Johnston wanted to be sure he understood it.
E.P. Alexander, Johnston chief artillery officer, was present when Johnston discussed his plans with Longstreet and he said of it later that "it is hard to imagine how any serious misunderstanding of such a simple movement could have ever taken place in a conversation that prolonged for hours."
Becuase Longstreet missunderstood Johnston's plan his forces marched off on Huger's route and got in the way of Huger's division and neither of those forces got engaged in the fighting. D.H. Hill got tired of waiting for orders to attack and attacked anyway and Johnston went to where Longstreet should have been fighting and got wounded then didn't command a major army again until the late winter of 1863.
So POD is this:
What if James Longstreet didn't missunderstand Johnston's orders? What effect would Johnston's plan for Seven Pines/Fair Oaks actually being used have on the campaign?