I think flavour text is not enough to make dungeon crawling interesting. The actual process of play has to be fun.
I think
@Manbearcat is able to make up dungeons as he goes along. I've made up (small) dungeons as I go along when GMing Burning Wheel. It depends very much on what the procedures are.
Yup.
So what actual dungeon crawling (not to be confused with sort of "mood-and-aesthetic-driven dungeoneering") requires is the following:
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Inventory, gear, and (non-spell) loadout has to matter. It has to matter as a limited inventory-based decision-point when you're preparing/loading out. It has to be an important component part of player decision-space when they're encountering obstacles/problem areas in the dungeon. It has to matter to the GM's mental workspace when they're considering consequences to action resolution/moves made while dealing with obstacles/problem areas and then it has to matter to players when dealing with those consequences ("crap...I've lost my torches/rations down the dark ravine while crossing the rope bridge...how much do you guys have...do we need to go after them?"). It has to matter when handing intra-dungeon inventory decisions (do I abandon these potentially important supplies to pilfer and pack out that bejeweled urn?).
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Procedures have to stable, sufficiently table-facing, and consequences have to be sufficiently telegraphed and with mechanical teeth. If players don't actually know how the game works...then they're not making informed, dungeoneering-based decisions. If players can't evaluate turn-based (or some kind of game tech akin to it) action economy and how that integrates with the overall delve attrition model + risk profile of taking this action vs that action or not taking action at all (and not just now....but longitudinally through the span of the delve)...well, then they're not actually playing a game of dungeoneer engaging dungeon crawl. They might be having fun, but they're engaging with a different activity that is more aesthetic, mood, performative, and experiential in nature.
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Simply put, D&D spellcasting needs to be brutally nerfed or every spell cast has to be costly or require a not-insignificant check (that will bring about interesting complications on a failure). D&D spellcasting is so brutally overpowered that it obviates the basic, constituent parts of dungeon crawling. The substrate of the crawl is enmurderated by D&D spellcasting. Inventory/gear loading out is undone, problem areas/obstacles are often obviated rather than engaged with, and basic paradigms of play (such as light and a ranger looking for a campsite for the group to recover in a dangerous place) are undone. In dungeon crawling games that work (Moldvay Basic, Torchbearer stand at the top of the heap), D&D spellcasting doesn't kill it stone dead.
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Units of play need to abstracted & unified sufficiently that they're user-friendly and table-time-friendly. Unit of play asymmetry or incoherency or just being too damn intricate and confusing (like weird action economy stratification rather than unification...or currency stratification along with compounding weight issues...and some supplies being brutally underpowered or not worthwhile because they weigh too much or are too difficult to load-out) is a killer for these games. Good decisions on all of these things need to be made up front so play hums along rather than stalls out (for any participant...players and GMs alike).
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Similar to the above, classes need to be functionally designed with all of the units of play as their collective guidepost. Otherwise, you get weird things like Rangers can't Ranger or Thieves can't Thief or some class does something thematically better than another class (like Fighter's cease to become the important "mule" because Wizards can trivially load out a, relatively uncostly, spell that makes for a better "mule" than the Fighter) because the design is borked.
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Adventuring Site design and guidance needs to be tightly constructed and well-communicated to GMs. # of Problem Areas/Obstacles and the hardship scaling for a dungeon of x, y, z size. How to spatially (points of ingress/egress, verticality) connect these Problem Areas/Obstacles and what a proper map entails (Keying, Point-Crawling or actual mapping if that is your thing). How to integrate rest areas but keep them dangerous. Functionally executable randomizer elements like Wandering Monsters/Twists/Random Encounters. How to invest the whole thing with theme. How to seed it with seductive treasure that must be agonized over (because it will be difficult to bring out without sacrifice). How to invest the thing with a variety of Obstacles archetypes, including some that are personal to specific PCs.
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Monsters and Conflict Resolution mechanics need to be interesting but agile, versatile, and sufficiently slim. Too much heft or lack of agility in this area becomes a problem for play.
One D&D has a lot of difficulty to take on if they want to make an actual dungeon crawling game. I wouldn't expect them to pull this off (nor be interested in doing so, because there are a sufficient number of trade-offs they would have to make and I don't think their user base is interested in those trade-offs TBH).
But it can be pulled off. And if its pulled off well, GMs who are proficient with the paradigm can make Small to Medium dungeons on the fly and execute them. Small dungeons are not prep-intensive (if the model is done well) at all and they're easy to execute. Medium dungeons on the fly can get tricky so they take a lot of hard-earned skill (and, again, a very capable system). Large dungeons? Impossible to perform on the fly in my opinion (regardless of how good the system or how proficient the GM is).