Advice on building a Megadungeon type game

[DragonBaneDM]

Hey guys!

So I'm close friends with a couple of DMs and players across four or five campaigns of a shared friend group who've always had the same issue: one or two of our players being unavailable each week, leaving us to stew for months at a time. To help fill the void, I've made the suggestion of a casual, rotating DM pick up type online game of 5E where we play in the same huge, ever expanding dungeon that partially repopulates each time the players leave. The size and makeup of the group doesn't matter, and you can run the game as long as you want.

It's a lofty ideal, but I want to make it happen. Here's some question I have before I start

How would you go about making a repopulating dungeon where players don't get bored by going through the same rooms and content over and over and not adding anything on?
Checkpoint Portals?
Multiple entrances?
New rooms and additions to the dungeon?
Rules to help new, level 1 characters catch
up really quickly to their cohorts?

What infrastructure should I set up in the forums beforehand?
Links to Monster Calculators and Combat
Calculators? (Yip yip!)
A unified treasure rolling table/treasure
guidelines used by all DMs?
Rules on shopping outside of the dungeon?

What general advice do you have from past experience/common sense that you'd give to someone trying to organize this?

[Tanarii]

If you really want to see everything that needs to go into proper mega-dungeon building for 5e, you probably want to ready Angry Gm's ongoing column. I only just started following this myself, and it's a very good structured approach.

Start here:
http://theangrygm.com/megadungeon-intro/

[MrFahrenheit]

Why do you have to go back through already-cleared rooms? Maybe each session, a new passageway is discovered ever deeper. This solves the repopulation problem. And since everyone's on board, any cheese factor is irrelevant, frankly.

Think along the lines of the old school dungeon hack games (to include the actual game titled Dugeon Hack).

[Telesto]

I did something like this before.

FFX sphere grid for layout
Thematic monsters for what the sphere is
Keys randomly generate to unlock key spheres
Empty nodes have keys/random loot
Certain rooms are good for rest, others have libraries on thematic topics

Enemies spawn at CR equal to APL, and bump up by 1 CR for every time the PCs return to the room. Level the enemies with the PCs

[Fighting_Ferret]

I would like to point out the video game Ultima Underworld 2: Labyrinth of Worlds, as a way to create a mega dungeon by using multiple worlds connected by a magical similarity and the influence of an over arching evil entity.

[Grod_The_Giant]

Have periodic portals to the next level of fit dungeon that are... not entirely reliable. You don't always come out through the right corresponding portal. Adventurers 1, 2, and 3 go in portal A, but adventurer 3 comes out portal B- and adventurer 6 comes out portal A with the rest. That way it's easy to rotate characters in and out. As a bonus, it gives everyone the opportunity to change characters when they feel like trying something new.

Also for what it's worth, you should consider still running games if only one or two people are missing. My rule is "more than half the group shows up and we'll play. "

[Osrogue]

Have a mix of enemies that will or won't come back to life. Demons and Devils poof back to their own plane, zombies and skeletons can reraise, there are always more rats and oozes, a cleric can heal its fallen comrades, animated objects might have a way to reassemble, some monsters are merely illusions by a master spellcaster, or the fabric that separates the multiverse is weak there, so the respawning monsters are being pulled in from other timelines/some of the monsters you fight are not from your material plane. I like this one, because it leads to fun like different rooms and optional areas may open up after the PCs have defeated certain enemies.