I would much rather see procedurally generated "Deep Dungeons" that you find in the overworld and then descend through. The deeper you go, the harder it gets.
Rewards would primarily come from chests guarded by powerful monsters using mechanics from random bosses from dungeons and raids. These rewards would be the previous raid tiers gear. (Example, when Deltascape V5+ is released, the "Deep Dungeons" would start dropping random V1 - V4 gear.
Materias and crafting materials would be secondary rewards from hidden chests, potentially also having gathering nodes spawn throughout, So that taking a gatherer would be beneficial.

Scale the difficulty to party size, and droprate of good loot appropriately.
Go in with 24 FC mates if you want to, and get a challenge appropriate to that many people. Alternatively, dive in with 2 friends and kill some time while catching up gear for alt Jobs etc.