Only problem is, it couldn't be pulled off without ending up with an utterly ridiculous plotline that would soil the reputation of a good game, like with happened with AC, DoC, and X-2. After all, Ultimecia is dead; you'd need to invent a new baddie, which Squareed up spectacularly in X-2, or revive her, which we definitely don't want; one Sephiroth is bad enough.
As for the topic, I'd say none, because none of them really lend themselves to a sequel. As for a prequel, I'd like to see Galuf and co. sealing away ExDeath 30 years before the events of FFV, or Braska's pilgrimage ten years before X started.