No. God no. FF9 is a stand alone. A sequel would just complicate things too much. Even a prequel about Zidane or Armerant would not sit right. The story came in, gave back stories where needed, said what needed to be said, did what needed done, and closed up nicely. If it could be the perfect sequel, it would be the greatest game ever. But there is very little to go by. The Iifa Tree? That may have been stopped with Necron (there are theories that Necron worked with Sooulcage, stopping Gaia souls while Soulcage supplemented Terra souls, maybe Necron is Soulcage's spirit). It is still pissed, but not wreck all of Gaia pissed: more like wreck anyone nearby. Vivi's kids? That is open to even more theories than Necron and Ozma put together, and one straight answer may send arguments of "well what about this?" Maybe an epilogue, or a short summary, or maybe even a CD Drama (Japan likes those) could finish wrapping things up with a nice bow on top, but a full fledged game is not possible without pulling random facts out of nowhere. It is like a 6 year old's fantasy, filled with "there's this too," and "actually ____ is ____ so ____".
Actually, after reviewing the Final Fantasy IX Ultimania, the Iifa Tree stopped its crazy stage about a week after Zidane went after Kuja, so it even wraps that up. Necron was described in the guide as being created from Kuja's negative emotions of fear, anger, and hate, just like Necron said. So he then just thinks the zero world is the most logical. He is 'eternal' because there is always emotions like that. Any other plot holes were cleared there.
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