Spoiler discussion so expect spoilers here.

So in a side-story quest chain in Revenant's Toll (which you obtain at lvl 54 after completing the lvl 54 MSQ) we meet 4 lalafells. After the side-story quest chain these 4 Lalafells run off and have not been seen since but they hinted that they had a large monster with them but they did not have bring due to issues this monster would cause in Toll.

I'm guessing by how the 3 Lalafells are greedy for treasure( except for 1 since she is always falling asleep) this may hint for a Future Hard mode Dungeon and this mysterious monster for that Hard Mode Dungeon Final boss.

Maybe Aurum Vale Hard mode.