The one thing that I find FFX lacking is sidequests. There were a few, but it seemed that the monster arena and blitzball were supposed to make up for the lack of sidequests, which I feel were cheap replacements.
One idea to flesh the game out a little more is that you could have gone on a sidequest for one rare monster(like a boss) in each of the areas to fill up the list of monsters(and killing it would give you ten of that monster).
What sidequest(s) would you have added to FFX?