These were very difficult for me, simply because of Zack. I've played FFVII and Dirge Of Cerberus, and seen AC; however, Zack barely appears in any of them, and I don't really have a firm (or any) grasp of his character. A lot of these also had to be Alternate Universes, a form I don't use often, because of canon events that limit the pairing.
I should probably say that in some of these (Constellation, definitely) I borrowed the motivation from Splintered Dreams, a Zack/Tifa fic that's on this site. Go read it; it's a far better example of this pairing.
Other than that, well, I hope you've enjoyed it.