1.0 instanced dungeons in ARR, no hard modes = suspicious...

[Balipu]

That's 3/5 explained away right there. Hullbreaker concludes a series of namedrops about Mistbeard's legacy reaching all the way back to 1.0, and I'm guessing the Keeper of the Lake will do something very similar - maybe it'll even contain yet another "lesser summon," the same way Snowcloak did despite being primarily framed around Shiva. And that's 5/5, leaving you only with the four 1.0 dungeons left out.

Well, Keeper of the Lake might tie in with Mistbeard as it features the same Kraken from Hullbreaker and Shastasha Hard. By the way I think that makes it the toughest creature in the game so far. Beware Bahamut, we have a Kraken.