The small issue is that you may have slightly overdone the exuberant badness of the notion.
For instance, dungeons are a bit large. Take a look at this map of the Ruins of Undermountain. Notice the rooms, and it's only the first level. Note that a page is about 250 words, so if you dealt with one room per page, you would be looking more at a level of a trilogy of fat novels, not a 50,000 slim tome.
Another thing is that the classic dungeon crawl is rather random. The dungeon has no unity. A novel, like any work of fiction, needs unity, as Aristotle observed long ago. It also needs variety. You can not even write a short story of a crawl (Jewel of the Tiger is a
If only that it's the maze constructed by a mad god to teach adventurers humility or something. A red dragon lair in a conquered dwarven kingdom would work, given kobold minions and watch wyverns and other underlings and enslaved dwarves -- encounters that require variety. Also, of course, it gives the story structure by giving it an aim and climax: kill the dragon!
Which, of course, does not preclude a weird, insane dungeon with its own alien mind in the "Dungeon as Underworld" trope. That, too, is a unity
But having a dungeon is half of it. The other big one is motives. Why are the foursome going it? Greed doesn't work very well as a motive in itself because it gives them no terminus. If they do it for the treasure, at what point do they said, "We did it"? Enough money to pay off the mortgage on the mansion? To bribe the king to free your father, ennoble you, give you a license to leave the kingdom (plus the added money to make a good appearance at court)? To show up the village that said you would never amount to anything? But there are others. A cleric or paladin could be going on orders to level up in case they are needed later, or there might be a reason why this is a quest. Any character might want revenge, or just to find out what happened to a family member. Any of them could be conscripted and forced -- perhaps there are hostages. Etc.
Hidden motives can enliven things. The fighter who secretly wants to be richer than anyone and so is never satisfied. Or the noble's bastard who falls in love with the wizard, who will whisk her away to his city of wizards and pass her off as a foreign commoner -- and such an offer completely contents her despite her aspiration of bribing to get a title.
Some of them have the advantage of the other motive: why THIS dungeon? If they are not ordered to it, why not other? Is it the only one within hailing distance with what they can afford? Is it that one of them bragged too much at the tavern? (Perhaps the local lord's son, and the others are coerced to go along -- but then you have to know why they don't shove him into the mouth of the umber hulk and run.) Specific treasure or monster in mind? Or perhaps there is a curse on all adventurers who do not venture here before they reach seventh level -- per, perhaps, the mad god.
Though after all this, I have to point out that these things can evolve during the course of the work. It may need more revision than the work where you start out knowing, but it's quite possible to find your four lumps of statistics turning into marvelously complex creatures half way through.