So glad to find a community of my OTP! ^_^
Here is some well written fics I've come across:
Approaching Ragnarok by paxnirvana.
A short sad pregame fic, the way the pairing is treated here is probaly the most plausible ways I've read.
Fix by
dev_chieftain.
Extremely well-written, smutty, and a very creepy Sephiroth. Be warned, there is non-con and it's dark.
Warmth in Winter's Heart by A-chan5.
Pregame, a mission gone badly. While not actual Seph/Zack, it could very well be preslash, with a few hints that would indicate it. Good interactions here.
One more thing, I read a story a while back that I'm dying to read again, except I've completely forgotten it's name, and any other thing that could tell me what it was. I just remember one scene, where there is a Sephiroth/Zack/Aeris love triangle thing going on, and the 3 are in someone's house awkwardly discussing it, and Aeris shocks them both by proposing to Sephiroth that they share Zack. Does that sound familiar to anyone? If you could tell me where to find it again I would be very grateful.