I don't think you really need a 'hook' per se to explore a newly discovered section of the Crystal Tower. Let's be real, they could just have Rammbroes page us and we'd come running. It's the centerpiece of the game at this point.
This is more your area of expertise, but I don't recall HoH all providing that much additional information to Crystal Tower/Allag lore, outside of a loose reference to Desch and Salina to tie it all together. And while I'm sure G'raha will make an appearance, I'm guessing this will be more centered around Krile and her father's history. Otherwise you'd just cheat and scan G'raha to unlock the next floor. You might get some bonus dialogue for completing HoH or Eureka, though.
They always design the story such that it's complete before the challenge levels start, and completing the whole thing is usually just for glamour (which PoTD had the best). If it's really important to you, though, I don't think it will be difficult to put together a group to get to the top. I have to resist temptation and defer it for two weeks this time around, but I know there will be people still running it out of interest.
