One thing I dont want it to be too cluttered. When I first played Baldurs Gate I explored every area before the City and eventually got to the city and saw how big it was and got fed up.
Things can be too big and overwhelming. Tons of quest givers asking you to do this that and the other all over the place, thats not fun.
It would be interesting to have a more focused game. Like for instance you can have only five quests at a time(seeing as thats roughly as much we can remember). Only once you've done one does a new one become potentially available. It would make managing large areas more manageable. Because in some games you want to explore a new area, but every few meters you end up dumped with a new quest, and so end up with about fifty quests.
Or maybe just get rid of the whole quest npc's dotted about all over the place? You need money so have to go to certain persons to get jobs similar to the Witcher. You wouldnt really wander around aimlessly, just hoping to stumble upon someone that wants you to do something.
It appears this game is going to have some purpose, your character is after something arent they? They wouldnt be too distracted from that, going off in the middle of their quest to find the grail to do a million random strangers a million random favours(there would at least have to be good reasons).
I think JRPGs do this well. They know that a big journey has to feel like a long journey. They love to have lots of little filler areas which seem to serve no purpose. Like stairs that seem to go on forever with nothing in them but it adds to the flavour. They can be large but they are still focused, the side quests flow into or slightly off of the main story.