My feeling is that all that speculation fails to address what the next campaign must accomplish in order to keep the Guild Wars world growing and vibrant.
This isn't a "Guild Wars is dying" thread <shudder> but a discussion about how the 4th chapter of Guild Wars can strengthen the overall game.
Here are some of my thoughts:
Guild Wars will not benefit from another chapter that spreads out its player base further. I believe that the next expansion should actually be set inside one or all of the previous chapters. This would keep the current population in place without initiating the usual diaspora to new content areas.
Not to say new content can't be introduced but it can be done using the existing maps and outposts. Obviously, there will have to be some enticements but I feel that there are many ways to add to the existing game that would attract people to buying a new chapter. (Auction Houses, for example.)
In effect, the new expansion would have in game "advertising' because previous campaign owners would be mingling with the new campaign owners.
This doesn't necessarily require that ANet:
-add new skills
-add new professions
Anet could simply choose to add 3 new RACES*, one for each previous chapter. Thus, using the existing environs in the game and ensuring that the population centers don't drift further apart.
(*NOTE: The new races don't have to be non-human, but Centaurs, Dwarves, Charr, seem to be obvious possibilities.)
In order to pull this off, the next expansion would be focused on current campaign owners instead of creating a standalone campaign as they have done before.
Additionally, ANet could roll all 3 chapters up together in the next expansion with the addition of the new races. This would be a good way to have a compilation release along with new content.
ANet doesn't have to give up their model of releasing standalone campaigns, but they need to have a "breather" every now and then in order to stabilize and centralize the player base of previous campaigns. Doing this every 3 chapters should be adequate.
Generally speaking:
The priority of next campaign should be placed on strengthening the existing game world. The expanding universe approach often causes a hollow world effect. The next campaign should focus inward but in a big and interesting way.
This doesn't have to leave new players/Guild Wars adopters out in the cold; it just needs to create new destinations within the existing game world.
Thoughts, ideas, criticisms welcome.

EDIT: Emphasis added, no text changed.