I'd go back to the Pharos Sirius mentality of dungeon design, with the intent of the highest level dungeons being that they are quite lethal on release. There's no way to stop age decay on content, other than instant kill mechanics, so my dungeon would have a few that go on to be notorious. I'd also do 6 bosses instead of 3. Let's put that 90 minute timer to the test, ehh?

Dungeon Name: Lunar Subterrane

My dungeon structure would be something akin to...

First trash pull is up to 5 packs of 3. Every odd numbered group in there would have a large monster that cleaves, the cleave applies a strong dot like windburn. Better watch out if you're not the tank. So the lead in would be what we're used to getting, a set path, maybe it spirals around and down though into a crater.

Boss 1: Rogue Namingway - Deceptively small with a full hit circle, making positionals harder to work for, and he would have heightened movement speed to make kiting it impossible. Tank buster would simply be, "Hum." It would be 250k magic damage unmitigated. His notorious instant kill would be a spell called, "Name Change." The Namingway swaps the names of the person closest to him with the person farthest away. This would probably take the form of a little rabbit faced debuff symbol on both players, bonus points if they can actually enact the temporary name change. Then, as it would usually be a tank that is closest, he would cast Hum. If the tank is the name swapped person, then it damages the person who shares the debuff with them instead. Beyond that he would have powerful raid wides that inflict bleed, and then he would also have a single target unique DOT that he randomly applies to each party member. The raid wide bleed would always take Esuna priority, requiring double Esuna from the healer for the unfortunate SOB getting the double bleed.

Second part of the dungeon would be the first boss's arena suddenly freefalling down a pit in the crater, functioning like an elevator fight. Trash would come out of the walls in the forms of red sandworms, bombs, and black flans with green eyes. The Flans would have Platinum Tongue cast, that needs silence or stun, and there would be lots of them, else they instant cast a powerful Thunder IV on the group that leaves an insanely powerful DOT on everyone. The red sandworms would buff with drain strikes, and they would hit hard, like Magitek Avengers from Ala Mhigo hard. And you'd be forced to contend with a group of 2 Worms and 3 Flans eventually, simultaneously. After the last trash monster dies, the second boss comes down on the platform instantly.

Boss 2: Evil Mask, big head, and it starts innately with a 10 second Reflect buff, specifically against magic. Reflected spells will deal double their normal potency to the caster. Through the fight it will switch its reflect stance every 30 seconds for 10 seconds, reflecting one damage type. Its tank busters will be called Holy and Nuke. Holy will grants a debuff that makes Nuke deal 999,999 damage, requiring the Tank to use their invulnerabilty, because it will follow Holy with Nuke after a few group wides and other shenanigans. After it casts Nuke on the tank, the tank will be inflicted with a debuff that caused Holy to deal 999,999, but then Evil Mask will cast True Wall, and then it will cast Holy on itself. Its reflected Holy will hit a party member, the one closest to the Evil Mask. So you can either let the tank die or prep another party member with shields and group shared mitigation. Evil Mask will do this again, until each party member has one debuff or the other, and then it will soft enrage, with first casting AOE Holy and then AOE Nuke.

After Evil Mask is dead, the arena will split in two, forcing the party in half for two trash packs on each side. The paths will converge slightly beyond the trash, and so if one group is faster than the other, they can loop back and help the other group, as the road blocks will open on both sides once one side has been cleared. Beyond that would be a couple more trash packs so you could enjoy being back in a group together again.

Boss 3: Lunasaur - This boss battle would begin at the player's discretion, but the Lunasaur would shamble its bones to the wall of the arena, turn, and spew Bad Breath across the entire arena, inflicting unavoidable status ailments onto the whole party. After that, Toxic Breath as a tank buster, adding an addtional poison DOT onto the tank. Then a raid wide called, "Moon Miasma" inflicting high damage and setting the poison timers from Bad Breath to 99 seconds. Beyond this is would auto attack, its autos being cleaves. At 50% another Lunasaur would spawn at the entrance and do another Bad Breath, and if not picked up by the tank during the cast, it would lock in its tank buster on whoever it got agro on. Then the only other thing to the fight would be, the Lunasaurs cannot die until both are at 0%

After this, the back path beyond would be a slide, and it would slide you directly into the 4th boss arena.

Boss 4: Ogopogo - A smaller black Leviathan looking sea serpent snakes its away out of the subterranean river that's around the arena, and its first move is Tidal Wave, only the Tidal Wave animation persists, and the fight now takes place underground, underwater. Everyone in the party gets a DOT from the crushing pressure of the water, but has no fear of drowning, since we are unable to drown. Ogopogo would be coiled up in the "air." Now targetable he does a no-cast bar tank cleave that is the same as Leviathan's but a lot stronger, so the party needs to make sure to not be near the tank. Occasionally, however, a tether will go from Ogopogo to the healer, but the tank can't get it due to the cleave, so a DPS must. This will be a similar attack to the A3S tether attacks, but instead of dealing levels of damage threatening to a tank, it will instead inflict a 5 second Throttle and Silence debuff. If not cleansed, the target will instantly die after the 5 seconds are up. Beyond that Ogopogo will do raid wides and he will gain a 33% haste stack every minute. After two haste stacks, the tether will resolve too fast to be picked up, so the Healer will die. This acts as a very soft enrage to the encounter.

After that you're flushed on down the tunnel, deep into a crystalline path. Gold Dragons, silver dragons, red giants, dark sages, and Artemis Maidens are the trash here. Sages have spells needing silence. The maidens have tank busters that can be stunned. The dragons randomly cast Blitz on party members, dealing moderately high lightning damage to non-tanks with a residual DOT. Three packs like that, and then Boss 5.

Boss 5: Lunarian's Soul - This boss would be sort of a fake out. It would start by spitting out its HP total by killing itself, but then quickly restoring itself to full HP. After that it would float backwards away from you, while ranged magic attacking whoever has highest agro. You have to chase after it to proceed. While you chase after it, adds called Lunarian's Memories appear, untargetable and launch AOEs at you. It seems like it will be a long annoying fight. It launches fairly non-threatening tank busters and raid wides every now and again, in addition to the add AOE spam. As it reaches 42% a black version of Bahamut swoops up from behind it and flare breaths the walk way, vaporizing the Lunarian's Soul, and applying a burn DOT to every party member. There's no treasure box for Lunarian's Soul itself, we now transition to Boss #6.

Boss 6 - Dark Bahamut - Black scales and red flare breaths instead of blue. Brings in an T13's fight mechanics as a recolored, rehash with megaflares and the towers and those black lightning buddy markers. Earthshakers with the puddles, but now they target every party member, since this is a 4-man. When he readies Teraflare at 1% instead of hiding in a neurolink, you interact with a fallen Evil Mask, which casts True Wall, and reflects Teraflare back at Dark Bahamut. This fight will be more show and substance.

And that's pretty much all I got cooked up for a really hard dungeon atm. Yeah, yeah, I know nothing like this would ever get implemented, especially not for four man, but if it did, then I'd want it implemented late phase of a catch up patch, gear that drops being between Tomestone gear and Savage gear in iLvl. So if it were implemented now, i495 gear from this dungeon. It'd also drop a single i490 weapon at the very end. All in all, if you knew what you were doing, it'd probably take about 45~50 minutes depending on your group's gear. It wouldn't be in a roulette until they made another dungeon like it. If a roulette ever did spring up, it'd be, "Brutal" roulette.