Playing around the new Gaan'Telet made me think. This is usualy bad (and corrosive) news, but I actually had a worthwhile thought this time around.
The new Gaan'Telet floors actually do a good job of making just about any run challenging (yes even rogues, wouldn't you know it), because they spawn a small variety of very different monsters - Meat Men everywhere in particular is pretty nasty, and you have goblins and gorgons for first strike, and wratihs and snakes for debuffs, and all in all - the over the curve guys can all meet their doom with an unlucky combo of "evil floor" + "evil monster composition".
Which makes the "fix broken abilities with very ugly specific matchups" philosophy of yore actually work.
The "Naga challenge" subdungeon uses a spawn mechanic which doesn't seem to be utilized anywhere else - the race of the enslaved guys reflects the player's race. I can't think of any other place where this happens, which is a bit odd.
EDIT: Ok so Darvin thought up a bunch, but very little obvious "challenges" based on it. Do read on
If higher end dungeons could have a "flexible spawn slot" or two (much like the obligatory undead slot), which would reflect the players choice of class, you could balance the dungeons to be difficult for the supermen (Monk, Rogue, Warlord, Assasin - the guys who are both "obviously above the curve" and "working as intended"). Then you could use those slots to moderately challenge other guys or make it hell for anyone with specific spawns.
Just stick a message with "We herd you was coming." Upon entering the dungeon, or have a "flexible slot monster" monster shout it out at the unfortunate cheezer. Players can prep for specific dungeons (with preps, dooh), why not the monsters? It's not like they don't have a reason to fear monks and rogues, they're bound to have herd of them by now.
If you can do this, you can take it a step further - you could make the VICIOUS dungeons silly difficult in this respect, but make the condition "fixable" by doing sidequests before you enter a VICIOUS dungeon. Doing sidequests could affect other things like getting a free prep, lowering a bosses hitpoints (or number of DP's or whatever), and make you feel as if you're really campaigning against say Namtar or Avatar.
Examples: Completing all the "run up" dungeons leading to Namtar's Lair with the priest could guarantee no Priest penalizing spawns in Namtar's Lair. Doing specific sidequests during the campaign for Naga City could unlock topside altars. Completing objectives on the run up to Dragon Isles could lower the Firstborn's hitpoints, and completing tougher ones could reduce the number of Dragonguards. And with 5 mini bosses in the Demonic Library you can go nuts.
It would make the game and kingdom progression really click. I don't know about the work involved (not sure if it'd be huge or not), but my game developer sense is telling me I'm sharing something that is worth money for free right now. Really interested what you guys think.