One of the ideas thrown around was the possibly to create special level "sets".
Start by building a set of special levels that should be guaranteed. I would put Hell's Arena, Chained Court, The Vaults, Unholy Cathedral, and the Mortuary in this position currently (given the way they're sort-of connected).
Now we create sets of special levels that are randomly determined upon level generation. Some examples:
1. Two sets: (The Wall, Halls of Carnage, Spider's Lair) and (Hell's Armory, City of Skulls, Lava Pits), then have the game randomly select one of those sets.
2. Three sets: (The Wall, City of Skulls), (Hell's Armory, Spider's Lair), and (Halls of Carnage, Lava Pits), then have the game randomly select two of those sets.
3. Two sets: (The Wall, Hell's Armory, Halls of Carnage) and (City of Skulls, Spider's Lair, Lava Pits), then have the game randomly select two levels from each set.
This would make every run have the same number of special levels, although which levels generate are still random to some degree. If we were to produce additional special levels, we could reorganize the sets to make sure there's still a good balance. In theory it would allow for a lot of different special levels (maybe even twice the number we already have) while keeping the number of special levels in a given run at a relatively stable number.
There are a few reasons I'd rather not simply stack on additional levels without discretion. First and foremost, it would throw the amount of experience you're gaining over time out of whack (and I would consider that one of the more balanced parts of the game right now) if you're the kind of person who will definitely go to every special level. Explorer/Conqueror runs would become increasingly harder (they're already difficult enough IMO). Easy special levels allow for reliable rewards, making the game easier overall, while hard special levels are often not nearly as hard once you've collected said rewards.
Though, if some of you want to suggest special levels, I would eagerly point you to the sandbox. You can build them yourself and, if a lot of people really like them, it improves the chance of adding them to the real game significantly.