I'd like to suggest that the Devs strongly consider the PS2 game Shin Megami Tensei: Nocturne and it's optional Labyrinth of Amala as inspiration for how to best implement the huge mega dungeon.
For those that haven't played Nocturne here is a quick overview of how it worked.
- The Dungeon first unlocks as a direct result of in game progress however entering it is strictly optional.
- The first level of the dungeon is VERY hard for the level it unlocks but entirely do-able. Successfully completing it immediately grants very useful items for that point of the game. However if you wait until you're stronger to complete it the rewards will become progressively less useful.
- Subsequent levels are unlocked by defeating difficult but (usually) optional bosses that become available as you progress through the game.
- If you enter a dungeon level as soon as you're able too (by defeating the optional boss as soon as he is available to be fought) then completing said level will be difficult but do-able and you will be rewarded appropriately.
The benefits for this I believe is as follows:
1. Every level has the potential to be challenging and rewarding -- but not overpowering -- for the time in game it becomes available.
2. It gives players basic gameplay ques as to when a new level is worth attempting (alternative to increasing difficulty of a dungeon without any real indication when you might consider trying it for a challenge), the player can then decide if they want a challenge and go immediately and not accidentally out level the difficulty curve of that level.
3. The game developers have an idea when a player will attempt the level at the earliest and may be able to work story progress both in and out of the dungeon.
4. Starting the dungeon at max level and then getting more difficult may sound fun -- but is there really that level of scalability for high level encounters that allows for 14 levels of increasing difficulty from the max level?
5. It gives the player a sense of progress as they get deeper into the dungeon and will look forward to the next time a dungeon level unlocks.
6. It prevents the dungeon by feeling like a grind by giving the player incentive to only tackle 1 or 2 levels at a time.
Interested in hearing others opinion on the subject.