I'd love a prequel as well. I don't mind sequels at all if they're done well [though sequels tend to flop for some reason], so normally I'd be interested to see what they'd come up with for using a sequel. However, as it's already been stated, FF VI wrapped up its story too well. Everyone had settled, and the essence of magic was utterly destroyed, so I'm not sure how they'd even find a way to bring magic back to make a decent story [granted, magic isn't required to have a functional RPG, but I'd imagine it being pretty boring without magic]. But then again, they keep bringing Sephiroth back over and over to keep getting his ass kicked by Cloud, time and time again, so I'm sure that if they did make a sequel to FF VI [highly unlikely], they would find some ludicrous explanation for magic to slide its way back into the world. And yes, the sequel would probably not be as good as the original. It would at least be enjoyable, but would probably be lesser in comparison unless they found some way to make the story really good.

But getting back to the notion of the prequel, I have to say that I'd be far more intrigued to see what the War of the Magi was like. And maybe at that point you could fight the Warring Triad at their full power [I'm reasoning that the Warring Triad you fight on Kefka's Tower is only a small fraction of their original power since he had drained most of it?].