They could have a tie in with the planned idea to be able to control a GC unit to enter the older dungeons.

Have it where instead those current dungeon items that are used to craft furniture are only found in 1 or 2 chest they could make it where you go in witha unit of 5, 4 being the NPCs you trained and the player as a Fisher/Botanist/Miner and you progress through the dungeon and go through the dungeons but rather than the main focus be reaching the end of the dungeon, it would be exploration and looking for the nodes to obtain unique materials.

With this though they could add a limit to how often it could be done, like a fatigue for the NPCs when you go on that type of dungeon Duty with them, this way if you wanted to lv with your NPCs in the older dungeons you could still do that and just make the other aspect a time based one that you can do maybe 2-3 times a day.