So, since i had the idea drawn up, i figured i would throw it in Sardelac so it wouldn't be wasted.
A recent minor earthquake has revealed something startling!
A long forgotten tunnel which was once thought to be used by smugglers has been revealed under the grounds of serenity temple.
King Adelbern has ordered a search party to explore this tunnel and to find out where it leads. You, as heroes of Tyria are to lead the contingent of explorers through this tunnel. Nobody knows where it leads or indeed if it is still inhabited. If anything does live down there, we can be pretty much assured that it hasn't seen the likes of humans for many generations and they may be hostile towards you.
I have arranged a personal guard to aid you on your way... they are waiting for you just inside the entrance.
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My idea is quite simple but would probably be the most exciting thing since sorrows furnace.
A tunnel starting at serenity temple is uncovered and it is our job to scout the tunnel to find it's end. It's an old smugglers tunnel which eventually ends at an as yet undiscovered outpost in the far south of maguuma. The tunnel would be immensely long and be filled with many types of monsters.
At the beginning, the monsters would be low-level such as level 4 devourers. In the middle, you could expect level 18's and finally NEW monsters of level 24 dominating the last third of the tunnel. This last third would be pretty large on it's own and is what would comprise of the best entertainment for ascended characters.
Low-level players would find this tunnel immensely fun because they could see how far they could get. Veteran players would think it was a walk in the park until they got to the later area. The whole tunnel would be filled with different terrain, perhaps even representing what might be above ground at that particular location.
For example, the tunnel walls under ascalon could be stained orange and have little interest. The areas underneath the mountains could have lots of stalagmites and chemicals from sorrows furnace. As the tunnel passes under the ocean, parts of it may be partially submerged, with water trickling down the walls. Perhaps it could be an underwater tunnel protected by a magical forcefield to hold back the water...you're the artists ArenaNet!!

Since the tunnel would be so huge, it might be wise to have underground outposts at one or two points to give people a primary objective and resting point. These could perhaps be called "smugglers rest" and "pirates sanctuary".
The eventual ending to the tunnel would be a new surface town called "The lost haven". In here you could find (hopefully) a new armourer for ascended characters, traders, storage etc etc.
The new town could even give rise to a new explorable zone dominated by a smoggy, dark and gruesome swamp, plagued by a fiendish breed of nastiness.
Perhaps in the future, this new area could give rise to yet more areas....who knows?
What would be the main objective behind the creation of such a tunnel? Why should ArenaNet spend time, money and resources on it?
Basically it would provide a new adventure because of it's sheer length, it would provide a source of escalating difficulty, it would have an objective at the end (armourer/outpost), it would have an outpost or two along the way (underground outposts would give waypoint objectives and a new experience since they are underground outposts) and it would have replayability.
Imagine the influx of people wanting to play this mammoth zone! How many people would try to "run" this tunnel and consider making it a profitable business?
"Running to The Lost Haven for 3000 gold. Pay 1000 at Smugglers, 1000 at Pirates and 1000 at Haven" - i can see it now.
Anyway, i hope you like the idea.