Well so far FFX only has two games ...
while FFVII has like a gazillion [ok. more like four more installments, and not all are games. I don't remember anymore and I'm not compelled to remember anyway].
In any case, if FFX-3 would be a prequel, then I say AYE!
It's a bit hard to spin a sequal out of what happened after FFX-2 ... haven't played it, but if there were loose ends from FFX, FFX-2 tied them all up with a good tight knot.
... Or you know, SE can always make a movie based on FFX instead
