So I have a homebew I've been running over a couple long campaigns.  I was wondering if I should include a mega dungeon, something that is a GIGANTIC crawl, purely optional.  For treasure, fun, xp, etc.  If you were in a long running campaign in a world with a large overarching plot but a big sandbox, would you WANT a mega dungeon, something you could go back to occasionally for the old school dungeon crawl experience, and to beef up your characters?  Or would it distract from a story driven game?

Speaking only for myself, no. I briefly DMed Dungeon of the Mad Mage and absolutely hated it so much that I couldn't even imagine playing a mega dungeon. The monotony of it is just brain melting.

Again, this is just me, so take it for what you will.

I appreciate the response.  Would it help if it were optional and you could leave and come back as you please?  Or just the very idea is unpalatable to you?

I would love it.

But then I am an old school player and although I prefer RP and story in general now, I still love the old school dungeon crawl. Plus, as a player, I make my own RP... I would find ways to RP about what was happening in the dungeon. I don't really need a DM to give me "a reason to RP."

Wonderful!  I have some ideas for the mega dungeon but before I delve too deep and spend dozens of hours, I wanted to see if it was something people would enjoy.  

As long as the mega dungeon isn't the entire campaign. I have the Mad Mage dungeon in my campaign world but it is optional. There is a guy at the entrance (in a tavern) who charges a toll to go in. The guy is different every so often as denizens from the dungeon make their way out and force a change in management.

So my players can go in if they want, but can also follow plot hooks I drop on them around the world.

That's kinda my plan.  Have it totally optional and can be accessed and left at will.  Well, left at certain points.  They will know it's incredibly difficult.  That'll be the whole point.  Treasure and exploration and XPs!

What do your players think of the idea? 

What WE would like or not like is really immaterial. It's what they would like.

A narrative driven world and a megadungeon crawl are two very different games. There's also the question as to why the players want to go into a megadungeon if there's a narrative taking place above ground. 

"Count Vilanus is about to send troops to suppress the village, and there's a monster wreaking havoc across the countryside. Therefore, let's go into the dungeon for a week or two to earn some XP so we can tackle those things."

Unless you can tie those two worlds together (i.e. Count Vilanus wants a MacGuffin from the megadungeon), you should probably just focus on one or the other. 

Why not megadungeon? I'd rather think of it as a series of connected dungeons that they have to periodically leave. But if you are going to use a mega dungeon instead of a series of short dungeons, make it so that there is something important to the narrative at the bottom and some reason why they can't leave it alone indefinitely. Optional mega dungeon -- meh, sure, if your world truly is an open world, then why not? If the players WANT to get to the bottom of a mega dungeon, then you should do that? Seems like a lot of prep for something that isn't absolutely going to be featured though.

My biggest difficulty with a megadungeon is I find them so implausible. One needs to have a good rationale for why one would exist.

An ancient necropolis can make sense, where a nearby city interred their dead, underground, defacto forming a network of burial sites over time.

Maybe it is the underground city, of an above ground city, such as to do commerce while ice and snow is above, or someother kind of environmental hazard. Or only wealthy people live above, maybe.

Whatever the reason, someone needs to explain why someone would spend the equivalent of billions of dollars to tunnel out a megadungeon.

How many D&D settings have flying cities/fortresses? The very nature of Forgotten Realms, for instance, is that the world we know is built upon the ruins of long gone civilisations who had impossible degrees of power to shape the material world and the very fabric of reality. Surely, megadungeons cannot be so implausible. If you need to, you could have it in a pocket dimension held in place in event horizon of a black hole and that would still be proper D&D. Maybe the mega dungeon was constructed over a million years by successive civilisations trying to capture the power dwelling within. We can get away with a lot as DMs without disrupting the suspension of disbelief.

Quote from Gvarayi >> 

Whatever the reason, someone needs to explain why someone would spend the equivalent of billions of dollars to tunnel out a megadungeon.

 "Because I have searched the multiverse for creatures unusual and dangerous, and let be known where these challenges could be found. Heroes cannot help themselves and will flock to this place, my place, to prove themselves against these creatures. And I want them to my friend. Every suffering, every death, every life that is lost and soul that leaves a body, is what I desire. Death and souls feed my hunger."

Quote from Wysperra >>

Quote from Gvarayi >> 

Whatever the reason, someone needs to explain why someone would spend the equivalent of billions of dollars to tunnel out a megadungeon.

 "Because I have searched the multiverse for creatures unusual and dangerous, and let be known where these challenges could be found. Heroes cannot help themselves and will flock to this place, my place, to prove themselves against these creatures. And I want them to my friend. Every suffering, every death, every life that is lost and soul that leaves a body, is what I desire. Death and souls feed my hunger."

 So, the megadungeon is a deadly amusement park?

It needs more work to make the concept gel, especially cultural (and legal) context, and the life and relationships of the founder. Ultimately, it needs to come across as plausible and sustainable.

Quote from Verenti >>

How many D&D settings have flying cities/fortresses? The very nature of Forgotten Realms, for instance, is that the world we know is built upon the ruins of long gone civilisations who had impossible degrees of power to shape the material world and the very fabric of reality. Surely, megadungeons cannot be so implausible. If you need to, you could have it in a pocket dimension held in place in event horizon of a black hole and that would still be proper D&D. Maybe the mega dungeon was constructed over a million years by successive civilisations trying to capture the power dwelling within. We can get away with a lot as DMs without disrupting the suspension of disbelief.

If the megadungeon is the ruins of an ancient civilization, buried underground:

• The layout of the dungeon would look more like an urban area, with homes, streets, and other functional buildings.

If so:

• Why is there still open space underground − why didnt it all collapse entirely?
• Alternatively, perhaps the city is a subterranean city − but why?

The Mega-Dungeon mindset can be a fun change of pace in a game, I even think of spellcasters and even the cantrips they choose will differ from an above world adventure. In the above world adventure the Light cantrip has some use, but nowhere near the use it would have in an underground setting where there is no light.

Sometimes it's fun to pull back on the RP and backstories and just go through a meat grinder.

Don't see why a megadungeon has to be completely separate from the main campaign arc. Part of a dungeon's treasure can be knowledge - books and maps. The ruins, statues, denizens, and magic items can also provide historical or cultural context. Sometimes chasing down these little side stories can be even more satisfying than pursuing the main one, and it really contributes to worldbuilding. Maybe uncovering the main story's BBEG's ancient original lair provides insight on how to beat him now. Or you offer an alliance with an evil being who wants to stop the the BBEG as much as you do. Just because it's optional doesn't mean it can't affect the main story.

This shouldn't be an either/or situation. It can be additive to your story.

Quote from Gvarayi >>

• Why is there still open space underground − why didnt it all collapse entirely?
• Alternatively, perhaps the city is a subterranean city − but why?

 Have not heard of San Francisco? Chicago? Venice? They all have extensive areas underground that are just sealed off and abandoned. 

Quote from Wysperra >>

Quote from Gvarayi >>

• Why is there still open space underground − why didnt it all collapse entirely?
• Alternatively, perhaps the city is a subterranean city − but why?

 Have not heard of San Francisco? Chicago? Venice? They all have extensive areas underground that are just sealed off and abandoned. 

Abandoned subway systems too.

Regarding the underground areas, I would try to get a map of one of them, and see what they look like. What kind of size are we talking about, what are its rooms or passage ways like, why are they the way they are? What function did they have?

I dont know about Venice. I am curious what its underground looks like. Heh, is it all underwater?

Then keep the reallife examples in mind, when designing a fantasy version of them.

I have a friend who wanted a fantasy Egypt setting, with a necropolis. I was able to get him archeological drawings of some reallife Egyptian examples. They worked well for the dungeon he had in mind. A megadungeon is slightly different, but the reallife examples help ground it in reality. It makes it feel more plausible, with more verisimilitude.