Is there any content in this game that is easy to miss, or locked out at any point in time? It's a fairly long game, with no new game+ (yet), so knowing what might be missed would help a lot going through the game the first time.
Yes, the Titan that houses Indol will become inaccessible at some point.
During the climb of the world tree, some time between chapter 8 and 9, Indol will become hostile. You will no longer be able to go back to Indol.
All other content should be obtainable at any point in time.