If it were to be set in Spira before Sin first attacked, then yes, I'd welcome a X-3. Imagine a huge open world game with sprawling cities and Machina lining the horizons. No more world map with an automated airship that just takes you from place to place. We could get a true feel for how the whole world was before the events that took place at the start of X, fully exploring the Metropolis that was Zanarkand and other parts of the world that we didn't get to see before Sin struck. So yes, I'd be for it if it were a prequel. But a sequel? I see no logical continuation of the story. I doubt they'd do a prequel though, so I voted undecided.