I reckon he's probably gone for good, this time.
After FFVII, Jenova was virtually obliterated, reduced to spiritual fragments that got flung far and wide by the Lifestream. Those fragments were mostly done away with when Geostigma was cured. The only remaining Jenova cells were in the likes of Cloud and other SOLDIERs. Jenova's head was the only 'useful' collection of cells that still existed, and that was destroyed after Sephiroth's defeat in AC.
As for Sephiroth... in the game, he was killed. His spirit lived on, but torn into three pieces. All of those pieces faded into nothingness at the end of AC too, so overall I'd say there's not enough left for Sephiroth to re-appear again.
No viable sources of Jenova cells. No coherent residue of Sephiroth's spirit. With neither body nor soul, it'd be awfully hard for him to 'get it together' yet again.

Anything's possible, of course, but the inevitable FFVII sequel will be sure to focus on Genesis Rhapsodos and his brand of mischief-making.