This information is from July, so I cannot testify for how things are now, or how they'll be at release. Take it with a grain of salt, and don't think any of this is gospel for what is to come. In general, ArenaNet is trying to be smart with their resources. Their new motto is basically: as big an impact, with as little resources possible. They re-using a LOT of past engine work and repurposing it in new and creative ways.
Expansion codename is Solthnys, the name of the deep sea dragon
Development started in mid 2019. Mike Z actually gave the green light for the expansion but soon left the studio after that.
There will not be any new mounts on release. Devs are very happy with the current amount.
However, a specific underwater mount was being discussed and mulled over. The Skimmer being able to dive also functions as data-collection on how people use a mount underwater, so that they can capitalise on emergent gameplay on the most efficient way if they do decide to release an underwater mount. This will probably be for season 6 living story, however.
New races will be released, Tengu and Largos
Only Tengu will be advertised. Largos will be a surprise when the expansion launches.
New races cannot complete past story lines.
The biggest concern was the ability to equip already released armour skins. They solved this problem very creatively by developing a special tool that adapts already-released armour skins to fit the new races as smoothly as possible. Largos models are basically the same as Humans and Sylvari, Tengu models borrow heavily from Charr skins. There will be some graphical hiccups, but the 70% most used armor skins have already been checked and properly adjusted.
They won't have any racial skills.
They'll be level 80 on creation.
Their respective home city can be found next to Lion's Arch (for the Tengu) and in Shing Jea Island (for the Largos)
There has been talk of revamping all racial skills (a popular idea was to make them into a trait line, so that every race could choose from every racial trait line: this would allow to make racial skills viable for PvM), but this fell out of the expansion scope. It's probably going to be wiped off the table completely.
Development was being done on Player-Owned Housing.
Together with the new races, this is part of the big selling point.
They re using the same script as they use for Guild Halls
Biggest issue here is trying to optimise the code so that every single player can have its own personal instance, since even Guild Halls are very resource intensive
Progress on this was very good, however. Beneficial work on this project MIGHT also mean that Guild Halls will have their furniture-cap expanded.
Scribing and Jewellery can go to 500 now.
Scribing: It will not be as expensive as the other ascended disciplines, to off-set how expensive the 0 to 400 progression is. Some items look very cool, like a huge statue of Aurene and Kuunavang.
Jewellery: Does not craft ascended trinkets, but uses their parts for crafting. See below for more information on this.
85% of the new expansion will be land-based. Underwater content is very scarce, but they're doing their best to make it as fun as possible. There is a lot of underwater areas to discover, however. But most of these area's will not have any events or bosses. They will be populated with throw-away mobs for the sake of collections and achievements.
There will be one big meta event that does take place underwater, with a big underwater boss that looks like some kind of eldritch horror. Lots of tentacles and eye-balls. Seemed to be a general from the sea dragon.
New elite specialisations will not use new weapons, but use underwater weapons instead.
There will be gen 2 underwater legendaries as well, not sure if they'll be finished on release though.
Elite specs are not finalised at all, but so far 5 out of 9 were as good as finished (design wise, they still have to be developed and balanced, etc.)
Elementalist: assassin-like spec that can dual wield pistols. Seems to use acrobatic utility skills. They'll probably be "tricks" or something. The mechanic revolved around using your elements as magical bullets and gunpowder, and you would have an F5 that functions as a kind of "residu bag" to catch all the droppings from your shot bullets. The F5 would have different effects depending on how often you used a specific element leading up to the bag being filled (so for example, using 6 fire skills and 2 air skills would result in a different F5 then using 3 fire skills, 2 earth and 2 water). They had only main-hand pistol at the start of design, but didn't think it would have the same feel as dual pistols.
Guardian: shaman-like spec, the virtues now become placeable totems with various effects. Utility skills would play into that, making your "virtue totems" pulse with specific effects. Meant to be less support-y than Firebrand apparently. Weapon is trident.
Necromancer: I really liked this. It was a mix between a witch and an archer, using short bow. Would work with area-of-effect cursed areas that had a variety of different functions and effects. Shroud was some kind of dobbler-mechanic, making a fleshy-shadowy version of yourself that would taunt your enemies. Your F-skills would change in skills that your dobbler would use. They used some kind of trap-like utility skills, but they weren't called "traps". They function like traps, though... But I guess they're supposed to be cursed locations that get triggered when enemies walk over them or whatever.
Mesmer: bard-like spec. Would work with music and flutes, didn't see a lot else. Think it also used trident.
Ranger: Beastmaster, seemed to use the scepter as some kind of "whip", and the utility skills would be similar to how (elementalist) minor elemental glyph worked before their rework
Warrior does use spear and Thief used harpoon gun. They were still deciding if they were giving Engineer harpoon gun or not, since they were having trouble finding a good spec for Engineer. Apparently they're floating the idea of an Alchemist-like specialisation, but they're not sure how to execute it properly.
Masteries revolve around different things. The only one I'm aware of is "spirit gems".
The Spirit Gem mastery line would work on unlocking how many slots you get access to (see below)
Spirit gems would be a new way of customising your build. They are items that can be used in a new tab in the hero menu. The interface of that new tab looks like a huge hexagram with lots of detail. The graphics shows orbs that looked similarly to those seen during Omadd's machine. Every corner of the hexagram was styled to the theme of an elder dragon. There are slots in this hexagram where you can put the spirit gems.
Spirit gems should not cause power creep, but would give you the choice to severely change your own play style dramatically. An example would be "Spirit Gem of Embers", which would change your bleeding application into Burning (but half duration). You could slot this spirit gem into the Primordus corner of the hexagram. Every corner of the hexagram has limited slots. The goal of spirit gems was to take a part of your combat, and change it into something else (so not just piling on to the combat mechanics, but effectively sacrificing an aspect for another, a "quid pro quo" of mechanics they called it.)
Not every profession can use every spirit gem. Arenanet can quickly change which spirit gem each profession has access to, so that they can quickly disable very distorted or overpowered gameplay that wasn't foreseen predicted.
Spirit gems are always tradable. Some are craftable with Jewellery.
Some spirit gems will be very popular, and these will therefore be a bit more rare. The point is to give more kinds of rewards in-game for people to hunt and trade. Give a better feeling of satisfaction when getting rewards.
Some spirit gems will change the graphics of your condi, so they're not all combat-related
Aura's will potentially be changed into spirit gems, but this was still being debated (since aura's should not take up a spot in the spirit gem interface, as they're mainly meant for combat effects). They're floating the idea of making a separate "island" on the hexagram for cosmetic spirit gems, which would mean aura's could easily be turned into spirit gems. But as I said, it's still being debated internally.
In general, the goal is to keep the average amount of DPS in the game the same as it is now, but they're prepared for expert players to find ways to boost DPS significantly. They'll only intervene when the DPS boost is too big, but they have a lot of levers to pull, such as traits, skills, utilities, etc.
Jewellery will go to 500
You need to disenchant ascended trinkets in order to get the main materials, globs of dark matter gets used a lot as well.
Most spirit gems that can be crafted will be good for the average player, and they include some aesthetic and fun spirit gems as well.
Raids and Fractals will still be scarce. IF they get made, they'll be aimed at the hardcore fractal player (in the form of challenge mode) or the hardcore raider. They tried to make the two previous raids more easy to lull more people into playing them (this wasn't a success however), but if another raid wing gets made, it'll probably be Dhuum-level (all the way) or harder. They said something in the lines of wanting to develop a raid boss encounter that would take more than 2 full days before it would get defeated for the first time ever. Even more days would be nice to see, since the devs love to sit in the TV room when a new raid gets completed, to see how players try to clear them out on Twitch.
PvP and WvW will get new reward tracks akin to the tracks when HoT got released.
They'll spend a bit more extra care on PvP and WvW rewards, so that those players don't feel left out. Truth is, they would love to build more toys for PvP and WvW, but they often don't have the resources for big changes.
Nothing from the lore is being revealed to anyone except for the developers who are building the missions. They're really tight-lipped on this. Obviously the story line will stop concerning the Elder Dragons, but from what I've seen, we're going to go in a "deeper" layer of the story line? Basically looking at the entities (if you can even call them that?) that formed the Elder Dragons, the Human gods and other divine characters. It doesn't seem like they'll go the World of Warcraft route, and start talking about the Uber Elder Dragons or whatever, but they seem to be heading towards establishing the creation mythos of the Guild Wars universe and toying with that lore. Speculating here, however.
Cantha looks stunning.
There are 6 new, huge maps (a little bit smaller than those of Path of Fire.) The world of Cantha hasn't changed an awful lot compared to how it was 250 years ago.
Echovald Forest is one of the huge maps, and looked a bit like a fully developed version of the forest-bit of Bitterfrost Frontier (but with less snow and winter-aspects). Huge trees, creepy mist, it's extremely immersive. Apparently they're working very closely with the audio-team here to seriously evoke that eerie/creepy feeling of being all alone in a haunted forest when you're walking through it.
Shing Jea Island has the underwater meta I spoke about earlier. The map's central point here is the monastery. This map has about 70% of all the underwater content found in the expansion.
Other maps are the Jade Sea (big focus on exploration), Kaineng City (big focus on meta events, basically the same workings as Drizzlewood Coast), and two new ones called "Yeti's Crown" in the south and "Great Luxozsh" in the east (which I haven't seen footage or gameplay off, but they told me it was a "nomadic city" build by the Luxons?)
Not a lot of new adventures being planned.
As always, lots of achievements.
They're putting extra focus on making good use of the new crafting materials. A lot of previous expansion-specific resources will also be used for new items & recipes. I wouldn't recommend hoarding them, they'll probably just go into single-digit-silver territory.
Expect gameplay footage (as a teaser) near the end of this year. The footage is already almost finished but they are planning the "hype train" very carefully since there is still a LOT to develop.
The footage shows the player character walking towards Kuunavang, who is lying on the floor (with some generic dialogue), and Kuunavang then looking into the camera. Then it shows parts of Echovald forest, and a scenic view of the underwater areas found in Shing Jea Island. It then continues with the camera "diving" deeper and deeper. It ends with an extremely creepy scene where you see a pitch black underwater cave (with some bubbles and small fish), and the background seems to show some HUGE and creepy shadow, that eventually materialises into a dragon, that is staring directly into the camera. You only see the dragon come into shape very slowly, but by the time your brain has put together the image, the video ends. The sound effects are seriously on-point for this one. I had cold shivers watching the footage, because it really evoked that "deep and creepy" feeling for me. Best teaser video they've made so far, by far. Kudos to whoever made that!
Expect the expansion to be released 2021, september.