We need something so sneaky that the gamer will go: "Holy sh...cow!!! What have I done?" when the plot is revealed. We need betrayal like none before...or something.

How about this:  In the Asian setting the Hero is tricked into supporting the wrong candidate for Emperor.  This candidate appears all good and virtuous, but is really manipulating some wicked stuff behind the scenes which you only find out near the end.

The Hero might have slaughtered enemies and conquered towns on behalf of this candidate only to find out he's perpetrated atrocities.  And in fact, the actual good guy was the one you thought was evil all along.

Btw, in QFG4 it wasn't obvious that Katrina was evil until near the end, IIRC.
 

Wasn't this kinda sorta already done in QFG?  In 2 you were enchanted by Ad Advis and retrieved the statue of Iblis for him, in 4 you were forced by Katrina to gather all the relics for the summoning.....only to break free and stop them at the last moment.

All the QFGs were awesome (my favorite was 2), but my only critique would be that from 2-5 the major plotlines were all pretty much the same: the evil bad guy is going to summon some terror to destroy the world and you have to stop him/her, whether it's Iblis, a Demon Lord with one messed up name, Avoozl, or the Dragon of Doom.  

Let's try to brainstorm some other cool plots.  From Sierra we have:

1. Evil witch puts curse on area which must be lifted by Hero.

2. Bad guy/girl wants to summon freakish monstrosity to rule world, possibly tricking Hero into helping, then has to be stopped by Hero.

What else can we add to this?

For the Norse setting we could consider having the Hero stop Ragnarok from destroying the world, but that might be too advanced for a low-to-mid-level hero on his second quest.

http://en.wikipedia.org/wiki/Ragnarok

If a Native American setting is being planned, I suggest an Inca or Aztec setting.  

Quetzalcoatl should make an appearance.  Stamina potions should be made from chocolatl .

As for the Norse setting, I hope to see the most famous elements of the mythology expressed, including the cosmology, Yggdrasil the World Tree, etc.  In QFG3 you had different races, like the Leopardmen, those Liontaurs, and the human Masai-like people, I forget what they were called.  In the Norse setting I think there should be a Frost Giant lair or stronghold, and a Dwarven city.  Dwarves should really be involved, since they're mostly of Nordic inspiration, I think.  If anything, the blacksmith should be a dwarf, and the Hero should be able to eventually by some cool weapons and armor from him.