Perhaps it already has been mentioned, but I, for one, would LOVE to see more interesting story / plotline in GW, closely interwoven into the game-play.
All of these are PvE suggestions and they are geared around:
---> having the players AFFECT the way the game is played based on past choices is a GOOD thing.
---> having a more immersive environment is a GOOD thing
---> replay-ability is a GREAT thing
My suggestions:
1. more tying multiple quests together with hidden flags to create story sub-arcs...
To a degree, this is already being done -- such as the attribute quest from Vanyi in Droknar's Forge or the Althea's Ghost and Ashes quests. However, what is lacking is better story arcs. For example: wouldn't it be interesting if back in the beginning of the story, in the Ruins of Ascalon, you were tasked to look for Vanyi and find out what happened to her and her father?
Then, during your travel through Northern Shiverpeaks, you might have come across some intriguing hints of her whereabouts.
Finally, you encountered her and the Mraaga demon in the later portion of the game. This would make a far more interesting sub-arc IMHO...
It also creates alot more continuity in the game. Instead of very episodic and choppy...

Also, the flags that are results of a quest should not be just "Completed vs. not Completed". Quest might have different results... A rescue mission might end up having the hostage killed. A retrieval mission might end up getting the wrong item. These will, in turn, generate a different effect later...
2. create more memorable characters and UTILIZE them
GW already has some interesting characters (Rurik) and memorable ones (Cynn, Devona, and Gwen). What the game lacks is interesting interactions with the characters and the ability to affect the character development through play.
Some examples of this:
A. You might have to complete a series of linked quest before you can get a particular henchman. if no one in the party has completed the quest, then the henchman won't join your party. So, to take the example from above -- Vanyi might be a henchie -- but unless you actually completed the quest and freed her, she won't join you.
Think about it -- there will likely be an assassin henchie in Chapter 2. Well, unless you befriend / aid her in a mission of some sort, she won't join you...
B. Depending on your completion of a certain character quests, the henchie might have different skill sets. For example, at one point, Lina (the protector monk) might have a quest to reach a hidden temple to Dwayna (you have to assemble a party with her in it). If you complete the quest, then everytime you use her as a henchie, she might have a special Elite protection prayer.
C. The henchies might take on different secondary professions base on their development in your story line. For example, if you took Lina into a nasty undead temple and uncovered a evil artifact. You have the option of giving the artifact to her. If this happens, then Lina might actually be Mo/N instead of a pure monk protector.
[Slightly off-topic technical item: For multiple human players, I suggest a vote based scheme --> i.e. you aggregate the quest results among the human players in the party. Majority wins out, with a default tie breaking scheme. It would also make sense to have a brief pop-up of a henchie's skills when the human players get one for the party, so one knows exactly what kind of henchie one is getting.]
3. Finally, have some sort of conversation tree that ties into the story arc / quests and the past actions of the players.
Even a few branches will make a huge improvement to the current "same story to everyone" approach better. If I am a healing monk and I just finished the quest to help some orphans. Then I might get one set of reaction from Alisia. In contrast, if I am a death necro who has just completed some quest involving digging up someone's grave then I should get a different reaction and different quest options from Alisia.
4. Miscellaneous:
---> playing a certain series of difficult quests and completing them in the right way gives you a +10% benefit when selling to the normal merchant.
---> playing a certain series of quest involving a dye merchant gives you access to a little panel that enable you to examine your dye mixture BEFORE you actually mix them.
etc. etc. The point is to TIE the gameplay to the story, so that we, as the players, feel INVOLVED.
Now, clearly, all of these suggestions will take some effort from Anet. But I think they are certainly feasible technically and there are BIG pay-offs:
----> even if you finished the game as a wa/mo, there is now a huge value for you to play the game again as a new wa/mo AGAIN. WHY? because you can go through the quests with different results, even different henchmens.
----> your action in game now matters. it is no longer the case when everything is just simplistic and black-and-white. you might have to think before you decide to do a quest a particular way.
----> yes, i understand that the main arc might have to remain the same (since cut-scenes and such are difficult to have multiples of). HOWEVER, there is no reason that the little quests cannot be more tied together with more slants toward character development of the NPCs.
Comments, criticisms? Am I just dreaming?
