The only true missable shit in FFX are the Al Bhed Primers found within Home (which you go to just over half way through the game, at the end of the Bikanel Desert). There are three in total and if you leave the area without getting them all, then you can't get them at all and therefore can't get the trophy. Other than that, there ARE the secret treasures you can get in each Cloister of Trials. You CAN technically re-do these to get them later, BUT two of them (Besaid and Macalania) are guarded by super-bosses, and getting all of them is a requirement for getting an optional summon, which itself is a requirement for getting ANOTHER optional summon. :maybe So yeah, just look up al bhed primer locations and how to get the Cloister treasures, nothing else is really missable.