Are there any hidden classes? (no blue exclamation mark)

You will receive the key in Chapter 6. When you're back in Grand Ship, you'll see a glowing object on the ground - pick it up. It's the Treasure Chest Key, and will unlock all the blue chests. They tend to carry the best items in the game, so remember where you found them and pick 'em up.

When you fully rebuild Norende you have access to some pretty sweet items, with the village shopkeepers occasionally sending you some of their stock as a gift (sadly, never weapons or armor). But besides that, you only get an extra Party Chat scene. No extra gifts as far as I can recall.