How are you handling experience for this? I presume most of it will be coming from quest rewards which ought to work. I cannot imagine a megadungeon campaign where characters are gleefully seeking out combat.
Anyway, how are you handling dungeon levels and the like? Are these levels going to suit only heroic tier play or is the whole dungeon going to serve all three tiers? I feel like it's the former but I have no sense of how Devil's Gullet (the environment seems nasty), Bleaker's Delve (a wizard did it allows for a lot of craziness), and the Underground Lake (could be relatively normal or balls-out chaotic if the Far Plane intrusion is great) will play out.
I had a tentative idea ( I even broke out a table) of each level having a range of monsters. Then I was going to throw certain quests in there as well. So there's XP for killing monsters and XP for quests.
Like, well-- in my mind, a quest might be "Get a copy of the dedication of Khazarl-Lance" and then, somewhere in the game (maybe through research or shaking down the right NPC in town) the players might find clues that the inscription was once carved on the base of the colossus. So they head out towards the colossus and have to deal with the spider-nest directly next to the inscription, or maybe they only get
part of the inscription (with the rest crumbled off) and then have to then consult the mongrelman shaman who has the best collection of ancient carvings from the rubble-heap.
In this way, the dungeon itself is kinda like a playground, and the quests lead PCs to specifically explore certain areas. And even in between quests, they have to deal with the creatures in areas they move through.
The entire thing would be heroic tier- The 12 dungeon areas having an overlapping range of 3 monsters levels. So the ruins have level 1-3 monsters, (mostly vermin) and then the catacombs have 2-4, and so on. The upper level range would be 15th or so.
One idea I had for a plot that leads PCs into the dungeon: The Orchid I picture as this giant venus flytrap flower thing which has a cult of worshippers, that -later in the dungeons life (once most of the place has been cleared..) eventually starts creating "pod people" in the nearby village/city (the cult lures townsfolk away to be ingested by the plant, which then regurgitates pods that are returned to town.. once the PCs figure it out, they have to fight their way to the last layer, wipe out the cultists and then fight the gargantuan orchid itself, which will be like a level 15 solo.