Some dungeon ideas, share yours!

[Altijacek]

Aztec/Mayan/Incan temple.

Start in a rainforest and fight a Couerlregina. Continue into that image above and fight a Silent Guardian, a big stone thing like the Adjudicator in Qarn, and then proceed into a pyramid and go from the inside to the top, where you fight Quetzalcoatl.

[Altijacek]

Having traveled to Istanbul, I would LOVE to see an abandoned version of it, as the sun rises, or sets. Also, Constantinople in AC Revelations was excellent, and a very accurate rendition of modern day Istanbul.

You'll start in an abandoned Merchant's street, something like Taksim Square. You'd fight a large giant like a cyclops. Then continue towards something like Galata Tower and climb it, and teleport to a mosque, where outside a group of Ottoman Military soldiers will be (heck they can even be called Turks as a reference to FFVII), and you'll fight wave after wave. Lastly you enter the Hagia Sophia and explore it and the underground where the recently discovered Gate to Hell is. There you will fight an Ahriman like boss since the Evil Eye is so popular in Turkey (can just be called Evil Eye in Turkish).

[DefendPopPunk]

Airship "dungeon". Fight your way through from the cargo bay or whatever, maybe your party was stowed away. Final boss on the deck, with beautiful Eorzea below you.

[Zetsumei_Tsunarashi]

I always thought inside the ruins of the Agrius would be a good dungeon.

[Kule]

Runaway train with a bomb on it headed to Garlemald like in FFVII or Lost Odyssey.

[Jinko]

As for the next Trial.

[Farryn]

These are really great ideas. I especially like the Istanbul one. Personally, my favorite dungeons in the game are the ones with complex mechanics and fun boss fights; while the new Expert dungeons are a bit easy I really enjoyed them and I'm ready for more challenges.
I think it would be fun to have a dungeon set in a Mako Reactor. I also think Costa Del Sol would be a nice place for a dungeon to be located.

[Kyan]

I want to enter a dungeon which is actually the mouth of a giant pudding! And we attack it from the inside! It's so squishy and purple!

...Ok, that's lame.

How about dungeons which relate to the beastmen?

[Enkidoh]

Having traveled to Istanbul, I would LOVE to see an abandoned version of it, as the sun rises, or sets. Also, Constantinople in AC Revelations was excellent, and a very accurate rendition of modern day Istanbul.

You'll start in an abandoned Merchant's street, something like Taksim Square. You'd fight a large giant like a cyclops. Then continue towards something like Galata Tower and climb it, and teleport to a mosque, where outside a group of Ottoman Military soldiers will be (heck they can even be called Turks as a reference to FFVII), and you'll fight wave after wave. Lastly you enter the Hagia Sophia and explore it and the underground where the recently discovered Gate to Hell is. There you will fight an Ahriman like boss since the Evil Eye is so popular in Turkey (can just be called Evil Eye in Turkish).

Personally it seems to me the Sil'dih Excavation Site in Central Thanalan is possibly planned to be a future dungeon, and given it's lore and background, would be exactly what you want (Sil'dih was Ul'dah's 'twin' city until the two went to war against each other over water, which Ul'dah eventually won due to Sil'dih resorting to necromancy to turn it's fallen soldiers into undead troops - naturally this backfired on Sil'dih horribly and resulted in Sil'dih being destroyed via a zombie apocalypse, and Ul'dah ended up burying the whole city.)

Now that it's starting to be excavated, it's clear SE has it as a possible dungeon location for the future - I admit I can't wait.

[Marxam]

I'm still hoping for a Cave of Trials in the desert somewhere. Those who played Star Ocean 2 know what I'm talking about.