This is something that I would love to see in the game and I think it would bring some much needed depth to the game.
I don't want open world dungeons, per se, because I don't think they could release enough space for everyone to be able to participate and still have good rewards, but how about something like this:
Bahamut's Scar
Bahamut's ravaging of Eorzea left several open wounds on its surface which contain mysterious and powerful enemies. Interlocking caverns and catacombs contain chambers and corridors which sunk beneath Eorzea's surface in the aftermath of the carnage . . . and some that rose from beneath the earth. Adventurers can expect to find both familiar and strange wonders and dangers alike in the caverns of Bahamut's Scar.
- Minimum iLVL requirement of 100 to enter Bahamut's Scar.
- A light party is required to enter, but players are allowed to bring a full party into Bahamut's Scar.
- Players will gain access to an instanced version of this dungeon, which will be physically the same each time (depending on point of entry, Limsa, Gridania, Ul'Dah), but will have randomized enemies.
- Players will face randomized encounters consisting of small to large groups of enemies found throughout Eorzea. Each enemy has a small change to drop gil, crafting materials, or materia. At the end of each encounter that will take place in a sealed chamber, there is a chance for anywhere between 1-4 chests to drop, containing anything from rare Glamor-only armor, to Extreme Primal Mounts, to Sands of Time, to Allagan Weapons, to little blue birds.
- Uncommonly, players will encounter larger enemies such as an Enraged Adamantoise. These encounters will involve monsters that have a large amount of hitpoints, whose attack power will scale to the number of players encounter them.
- Each encounter that you face in Bahamut's Scar will increase in difficulty and reward until your entire party is defeated.
- Players are free to change jobs at any time they are not in combat in Bahamut's Scar.
- A new gearset of iLVL 105 armor, accessories, and weapons will begin to have the chance to drop after 5 successful encounters; the more successful you are, the higher your changes to obtain this gear.
- At all times, there is a chance that the party may trigger several different environmental dangers, including but not limited to falling ceilings (think DD second boss), electronic discharges (think wave AEs from T2 and T9), and new enemies appearing from Aether disruptions!
If you don't want to read the below explanation in brown, TL;DR = something repeatable that's randomized and worth doing.
I'll leave the rest up to your imagination, but something repeatable with random elements and a wide variety of rewards would be something that would keep me logging in a lot more often. Being allowed a party size of 4-8 would make this a quick, "/fc Hey, anyone wanna do Scar? 6 people? Cool, let's go." instead of making a PF and waiting for 40 minutes for exactly 2 tanks 2 healers 1 bard 1 smn and 2 dps to hop in. Job changing in the dungeon would make for some fun - that SMN you got suck? "Hey man, do you have a healer? Switch over, I'll go to MNK that might work better."
When your party wipes, you wake up outside the dungeon and you are done, give it a 2 hour lockout and make the first 5 encounters worth while for the gil and crafting mats alone. Just make it so that you wouldn't mind going in with newer players to get those rewards, but if you can go in with your static, you have a chance to make it 11-12 encounters is and get 2 sands, an i95 Allagan weap, and an extreme mount before you die.
Encounters 1-5 would be pretty easy, with 1 being like an Alex chest and 5 being something like 2 A-rank hunts at once. 6 would start in with scaled down S-ranks for your rare boss encounters and something akin to 20 level 55 Dullahans spawning 10 seconds apart for your regular encounters. Maybe you run into Aclepius and 6 Hygeias on encounter 8 with a 4 minute death-clock.
I personally think this would be mega-fun, I know it would probably be a lot of dev time, but I'd be willing to fore-go content like chocobo leveling and the useless 4-man dungeons we get with each patch to see something like this in 2.6 or 3.0