A more severe example of this is when I finally managed to take down the Vor Armory (including the level 100+ Ds!), and then went to Briagh's lair. Major letdown. And then I went through the portal back to Middle Earth to do the new old dungeons, and the monsters there were in the high 30s. I was level 50, so obviously I wasn't looking for experience, but I did want a challenge. The most fun I had on my berserker was taking down those totally unfair multi-hued worms in the Armory. Challenge = fun, no challenge = no fun. Revisiting Middle Earth turned out to be quite boring because of this (and because the dungeons were still explored; I'd make it so the dungeons are totally regenerated at a high level with new vaults and all). Once a character hits 50, every enemy he fights should be at least 50 to keep things fun.
In general terms, what we want is ever-growing challenges thrown at the player. While the player might perceive the occasional decrease in the game's difficulty (after finding Ringil, for example), the actual difficulty of successive dungeons and monsters should always be increasing.
I'm having a difficult time coming up with a solution that I like. The purpose of capping the levels in dungeons is to indirectly cap the level of the character at five above that. If all dungeons simply level to the character, then we need some new way of capping the level of the character so they can't just rack up fifty levels before even leaving Middle Earth.
What if we just said something like "You can't exceed level ten until you kill either the Shade of Angmar or Bill. You can't exceed level 20 until you kill three of the following: Sandworm Queen, Minotaur, Rantha, Old Man Willow. You can't exceed level 30 until you kill the Master." We could also do a simpler version: cap the character's level at 30 until they reach the Far East. Both of these feel very contrived and make little sense, though they would have the effect I'm looking for, I think.
Something else I considered was tying level gain much more closely to boss kills than it is currently, and de-emphasizing non-boss kills. Maybe every monster type has a finite pool of experience you can gain from it. For example, you can only gain one or two levels throughout the game by killing worm masses, but killing any boss gains you an automatic level. This would need a lot of testing and tuning, but it would sort of cap levels. This seems too complicated, but I really like the idea of emphasizing boss kills. A possibly simpler system would be to implement diminishing returns for each monster. You get full experience for your first stone troll kill, 90% for your second one, and so on, decreasing linearly down to zero.
I'm stuck. Somebody suggest something brilliant! You get a bonus if you make it even harder to reach level 50 than it is now
(eg, normal player level cap is 40, but the cap increases by one for each Orb of Linaniil you find, which are of course dropped only by bosses, so you really have to dig through the side content if you want to be 50 for the Istari fight.)