I read the script for the FF7ac trailer shown at the SE party and it might kinda hint that he might be back...mebe but it could still be a flash back...
(SPOILER) [Cloud and Sephiroth are shown fighting on the dome-shaped ruin. Cloud speaks to Sephiroth standing at the top of the dome]

Cloud: What is it you want?

Sephiroth, Masamune in left hand, raises his right hand high into the air. A swirling thing starts to appear around him, gradually growing and growing until it's huge]
[Rubble crumbles inside this giant cyclone. Cloud and Sephiroth fight it out in-between the rubble]

Cloud: What's going to happen to the planet?
Sephiroth: That... will be for you to decide

For the whole translation you can go to http://www.adventchildren.net/ff7ac/...iler_party.htm

For some reason I swear hearing(reading) that in the game but......mebe I'm just imagining things since I've read this so many times.