And I'm going to assume they have at LEAST a vague idea of what Chapter 5 is going to be.
So I'm not going to bother with those, I'm going straight to a suggestion for chapter 6!

Name: Guild Wars Chapter Six: Radiance
Setting: Archipelago + Underwater
New Professions:
Illuminator, contraction "I", aura color bright yellow. A caster class that is based upon enchantments, environmental effects (basically wards), and "scroll spells" that have long casting times but are potent and are classified as skills so they are harder to interrupt.
The Illuminators are the scribes and scholars of the Margonite people. While often seen buried in volumes of ancient texts or scrolls of their own creation, the Illuminators are never the less an order that are not afraid to put mind over matter in the midst of combat for the sake of their allies.
Aspirant, contraction "Ap", aura color dark purple. A melee caster class that, while initially frail, is very proficient at debuffing the opponent with the added effect of stealing a bonus which works only against that particular debuffed oppenent (or opponents) for the duration of the debuff. Would work well with the other melee "caster", Assassin, if combined with its "Siphon" spells.
The Aspirants are the most zealous order of Margonite priests who seek to unite the followers of the "True" gods (Balthazar, Dwayna, Melandru, Grenth, Lyssa) to stand against the ever present threat of darkness and chaos. They have mastered the techniques of drawing spiritual energies into themselves from their environment and foes in order to channel its power within their own bodies or into those of their team mates.
Theme: Margonite theme! You remember the Margonites? They were mentioned in the same way the Elonians were in the Crystal Desert. They build those structures that look like boats. Based on their obvious affiliation with all things marine, I figure their homeland is the perfect setting for an underwater adventure.
So here's my proposed story: the Margonite homeland is under vague threat that slowly becomes more clear just like every other stupid storyline so far. Only to make it less stupid, let's make the main villain something more... well... villainous than an undead vizier with a sexy accent and a confused spirit warrior who needs to practice his evil laugh.
Like, say, there have begun to be signs of evil activity off the coast of the Margonite homeland archipelago. There have begun to be small incursions of monsters and demons on the cities of Margonda (made up name for Margonite homeland) from somewhere off the coast. It is learned that there was once an ancient city populated by exiles from civilizations across the globe who congregated on an island just separated from the island chain of Margonda. These exiles all had one thing in common that bound them together, they had been exiled for worshipping the dark god of chaos, Menzies (Balthazer's half-brother as those of you who go to the Fissure of Woe already know). Their city was destroyed when the Margonites discovered they were attempting to summon the dark god himself into the mortal realm. Empowered with the will of the gods, the Margonites where able to invade and successfully smite the city of chaos, Menzoran, off the face of the globe using powerful scrolls imbued with the wrath of Balthazar who wished his half brother to never gain a foothold in the realm of mortals.
Now, many years after the city of chaos sank beneath the waves and the dark ceremony was interrupted, it is nearly forgotten by all but the most devout scholars of the Margonite people. However, the recent incursions of monsters seem to be seeking ancient artifacts and texts of great power, and it becomes clear during the first missions that they are after a sealed tome of chaos, stolen from the high priests of Menzies during the raid, that is fiercely guarded by the Margonite people as it is not yet within their power to destroy its cursed pages. The early missions within the Margonite cities, libraries, and temples, are ultimately a failure in that the tome is successfully stolen by the followers of Menzies who plan to use it to complete the unfinished summoning ritual.
When the thieves are pursued by the heroes, it is learned that there will be much difficulty in retrieving the cursed tome. The dark forces have long ago seeped into the sunken halls of the city, unbeknownest to the averted eyes of the gods, and begun to revive the bastion of evil known as Menzoran. The heroes must first figure out a way to survive underwater (kind of like the ascension/closer to the stars) so that they can then find a way to break through the magical barrier surrounding the sunken city and stop the forces of chaos before they are able to summon the dark god himself (obviously you are too late in the final mission and a mortal incarnation of Menzies himself is the final boss).
So the first missions will be learning what is going on then trying to stop the thieves of the cursed tome of Menzies. Then the next group of missions will be seeking and completing the methods of gaining the ability to survive underwater, Imbuement. The heroes become Imbued with the essence of a whirlpool beast by drawing it out and bathing in its blood, allowing you to move and breath freely underwater. Then the next group of missions will be finding a way to penetrate the magical barrier of sunken Menzoran, by assaulting the heavily guarded spires that power the barrier, resulting in the barrier dropping and the city resurfacing. Finally the last group of missions will be the raiding of Menzoran and the defeat of Menzies along with the tome of chaos.
You like? You hate? You feel so apathetic you don't want to respond? Let me know! Unless it's the third one, I guess...