Simple signed...
How to implement ?
Its simple ... put in in a the Main Towns at a location a big Library, where the player (library is instanced) will find on a little stone pillar a huge wide open Book.
When you click then that Book, a Window will pop up, where the player can then choose between 5 points:
Prophecies
Factions
Nightfall
Eye of the North (including Dungeon Quests)
Core/Events
In regard of which page the player chooses then, the player gets then displayed in a List all Quests of the chosen Campaign/Add on.
The Book will show all quests, that your Character hasn't done yet greyed out.
Quests, that are finished will be in white with a green hook at their side.
This way, when ever you want to know, if you missed a quest somewhere, just go to say for example Kaineng's imperial Library, look at the Book of almighty Magic & Wisdom "Grimoire", that lies in there and works for the Game as Quest Log Book.
Imo perfect solution, that would give the game also some kind of little RPG Flair. Just only soem kind of pop up windor, that you get by clicking just only a button would be boring XD
Coool looking Book, like this

< Grimoire
PS: When you choose then a Page, the Quests would be sorted then by the bigger Areas and alphabetically under those Points, like in the L-Tab List