Disagree WoW created vanilla Blackrock Depths, Stratholme, Molten Core, and other huge nonlinear dungeon zones like that. But yeah on the topic of BRD...

Blackrock Depths was a beautiful nonlinear dungeon full of quest objectives, unlocking doors, and tons of optional bosses. It was also home to a raid instance entrance and a vital forge deep within for crafting epics. It had a lot of verticality so by virtue of that the map was awful and unhelpful. I didn't play vanilla, but I hear a top to bottom clear took 4-6 hours to get to the final boss Thaurissan, and even if you just wanted to zerg to Thaurissan it took at least an hour assuming no one got lost.

It was a masterpiece of dungeon engineering but way too damn long and involved for an instanced dungeon. The only way I could see this working in FF's more bite-size content style would be sort of like Diadem - you start at a certain point in a BRD-size dungeon and are given an objective nearby to clear, then you get there.