All I want is a mission where you quickly play through the main story bits of Zack because some of those moments in Crisis Core are fantastic (Such as the Red Dead Redemption-style end, three years prior to that game). Maybe portray it as Cloud getting to understand the weird mentality he now has after that switch-over (for want of a better word) from Zack to him. Maybe just have them like those Laguna levels in FFVIII.
Heck, I think they could probably cover Crisis Core in a single opening cinematic like how they sped through the story of KH:Chain of Memories in the opening-credits scene of Kingdom Hearts II.
The scene you spoiler tagged was actually in the original game. It was difficult to find though. If I remember correctly you have to go to the mansion basement after Cloud's memory is restored.