So naturally the answer is Extreme Dungeons that increase the difficulty for a group past a certain gear threshold. Maybe keep the loot the same or change its colour but give more tomes at the end? it doesn't even have to be a story sequel/continuation rather than an option to keep endgame dungeons relevant when level scaling cannot be employed? We all reach the point regardless of skill level where we crave a greater challenge when we can faceroll our way through content to the point mechanics can be ignored or outright out dps'ed by the party. Thats not fun, just an expeditious way to farm tomestones. Thus my prediction for dungeons is an extreme mode roulette replacing the current max tier content dungeon when you reach a certain item level, perhaps include a stone sky sea test or some sort or a rework of the hall of the novice almost in the vein of World of Warcrafts Proving Grounds system that tests you for your role? This gives people more challenges without needing to make new dungeons or shoe horn another 24 man alliance raid in too soon between raid tears which might make the 8 mans stay relevant to casual pick up groups for longer?
My second prediction would be options/routes in dungeons. In HW they added more "heres a treasure if you want to risk another mob pull for it" with marketplace approved Minions up for grabs. Compound this with the added the concepts behind places like Deep Dungeon and Aquapolis with their 'choose your own path to endgame' i expect this ethos to show up in Dungeons as well. Because lets be honest one problem with games like FFXIV and the games it borrows from like World of Warcraft in particular is linear dungeons. So with so many dungeons already having "kill mob for key to unlock a door to progression" mechanics why not give the player a branching path? perhaps one leads to a mage boss and the other a magitek war machine? perhaps one has less mobs but more environmental hazards?
Eventually the userbase would figure out an optimum route and then you could argue it then becomes as linear as it is now and would waste resources but then isn't that the entire design ethos behind Hard Mode dungeon layouts to begin with? I imagine its not as likely as Extreme Mode Dungeons but i think this may also be a way we see a dungeon shake up.
My third and final expectation is an increase in dungeons not tied to the main story questline. We have seen already that 3.0 put beginners off by being so MSQ mandatory and when people crave variety this seems like a poor gating mechanics even if you love the story like i do. So i believe things like Neverreap were the beginnings of some dungeons completely unrelated to the main story. After all we are seasoned adventurers right? surely if there was a dungeon style event unconnected to our 4.0 quest we would still be called upon to help right? I imagine there will be more Haukke manor "out in the woods" dungeons in 4.0 that will be optional to find and will not lock you out of the roulette. Perhaps with their own side stories, perhaps they show up in the random selection but if you go there and get the quest you get some glamour gear for the trouble? It would really feel more like you are being rewarded for exploring and not just sticking to the rails of your themepark ride and with things like Deep Dungeon being a new example of "because its there" questing and adventuring thats proved very popular i can see this impacting the choice in some Dungeon placement and access in the leveling process for 4.0
But of course new dungeons are not the only given in FFXIV are they? There also comes new Hard Mode dungeons that see the Adventurers revisit an older dungeon where time and the story have progressed in your absence and either directly or indirectly your actions have lead to new tenants moving in and setting up shop for you to deal with. So as well as looking forward we must also look back at what places shall we revisit in 4.0? here are my predictions:
Why: Why indeed. The Dragonsong War is over right? Nidhoggs gone and theres no story left untold here right?
Not entirely. You ever pay attention to the final area of this Dungeon? Ever stop before heading up the final ramp and avert your gaze from the looming loot pinata and look down?
Probably not but if you have you would notice that the bare depths of the Aery are in fact a layer cake of Allagan technology right out of Azys Lla and that is never brought up in the story, has nothing to do with the dungeon and remains an untapped vein of possible dungeon story. Because we have seen that kind of tech in two places. The Binding Coil that was contained in Dalamund and in Azys Lla where Tiamat is imprisoned and a currently unused floating installation named a "biohazard containment unit" filled with parts of dragons in tanks of fluid. Who is currently chasing such tech? The Garlean Empire. With Niddhogs brood dispersed and the Aery now abandoned it is the perfect place for Allagan tech hungry Legion treasure hunters to avoid the adventurers and Scions in Azys Lla and attempt to capture some tech that its fair to say the Dragons, The Ishgardian people and the Scions would not want to see them get a hold of. Of all the Heavensward Dungeons i believe this is the one most likely to be revisited in a future Hard Mode story.
The Aetherochemical Research Facility Hard Mode
Why: This again is a dungeon that seems to have run its course with its story exhausted and no possible hooks left until you go back and look at what actually happens in the dungeon.
Remember the second fight? the transforming boss in the Bioweapons Testing facility? most players assume it is just a dormant weapon not shipped out for use that you accidentally wander into. However take a look at its often ignored dialogue. When it transforms into a Cobra it states "Testing Anti Cobra capabilities" which doesnt makes sense for a Cobra to be doing that. It does however make sense if it is in fact your 'Anti Cobra Capabilities' being tested. The boss is not a Bio weapon, it is the machine that tests the products viability before approving the product created in the labs before it.
So you defeat the boss and if you stop to notice its dying words instead of rushing for the elevator it says the following: Combat abilities - Outstanding. Approved for mass reproduction and again if this was talking about the boss it wouldn't make sense. It just lost the battle and clearly is not the 'Outstanding' product here. You are. You have just been approved for Mass reproduction in a facility which immediately after this area shows it has Allagan cloning facilities. You were so busy chasing after the Ascians and trying to stop Thordan you fail to notice this ancient facility has recognised your combat prowess and has began mass cloning of you and your party. You defeat the Ascians, King Thordan and the Knights of the Round and leave the facility. In the Dark. Unmanned. Chugging along and starting up those cloning machines ready to make a new martial force for a war long since ended and forgotten. The warriors of light are being mass reproduced here, unknown to the world and eventually they will make their presence known and who else can be called in to put them down or reason with them?
Just a theory mind you. However that dialogue is there for a reason. I think Yoshi has plans for this facility and Azys Lla to be used again, its story is not over yet.
The Great Gubal Library
Why: So far each new wave of Hard Modes has seen a "Because why not?" dungeon chosen less for lore reasons and more because Yoshi's team want to have some fun with an idea. In terms of the Heavensward Dungeons i can think of no more obvious candidate than the Great Gubal Library dungeon.
A colossal repository of knowledge and unexplored corridors and secret passages could house any number of demons, magical tomes and if they take some more FFV inspiration even some ancient technology far more advanced than the Empires Magitek waiting to be woken up by explorers taking a wrong turn down its labyrinthian corridors.
Very few lore reasons for this one but i can't see the team giving up another chance to visit a place like this, perhaps to looks for more information to heal Yshtola or save Minfilia from the Lifestream?
Dzemael Darkhold Hard Made
Why: With the Heavenward dungeons covered you must also look back to the A Realm Reborn dungeons that did not get any love in 2.0 or 3.0 in terms of revisits and the first dungeon that springs to mind is the winding caves and demon infested pits of the House Dzemaels forsaken mining complex in Coerthas Central Highlands.
We have cleared the Aurumn Vale of its monster infestation, helped the Ishgardians retake the Stone Vigil but in this instance we merely took out the strongest demons we cold find in what corridors we could explore. However this is a massive complex and as we pass through the largest open areas we can see multiple bridges and pathways we cannot currently access leading even deeper into the complex. We never solved the source of the problem in this Dungeon. We cleared the strongest monsters and the corrupted Crystals but what was corrupting them? did it leave? or is it waiting for us deeper in the dark?
Snowcloak Hard Mode
Why: One of the few places never truly explored upon in 2.0 onwards was the area of Coerthas inhabited by the Gigas Beastmen, the tribe that worships Shiva as their patron Primal. With Ysayle and her Heretics gone and no forces left to attempt to resummon Shiva to help fuel Zodiarks birth into the realm outside the void its a given the Ascians will once again return to driving the Gigas to summoning their Primal once again. With the Snowcloak remaining one of the few other pre 3.0 dungeons never revisited and its gorgeous theme and music only remaining relevant as part of the stage 2 anima relic step it makes sense that Snowcloak returns as a Hard Mode in 4.0 as the Ascians teach the Gigas to summon a very different Shiva than the Moggle Mog style interpretation/idea conjured up by 'Lady Iceheart' in the Heart of Ice content patch of ARR.
These are just my rough predictions for dungeons in 4.0 but the only way we know is when Yoshida and the team reveal them, perhaps at fan fest or maybe not till the expansions release. Till then all we can do is play the guessing game, but its fun to do while we wait eh?