The preliminary information on Advent Children suggests that Zack might be mentioned in the film, maybe he'll appear in flashback? We've already seen screenshots of Cloud visiting Zack's gravesite, with the Buster Sword planted in the gound as a mark of honour.

A prequel to FFVII, featuring Zack, Sephiroth, and Elmyra's husband fighting in the Wutai War would be the best ever, but that's little more than a dream I fear.