I was also thinking something similar to a swamp. Except that I'd add in some suggestions that:
-The "swamp" part is an outdoor area, and many of the effects are treated as weather/wilderness/survival related checks, giving some nice benefits to classes with wilderness class powers that are otherwise pretty useless.
-Instead of making the boss a tank in the form you're suggesting (which, IMHO, we have several similar bosses already), I'd suggest something more like this: Boss has fairly modest HP, but very, very high regeneration rate--he can recover full HP every turn or two (sort of like Jharod, but, obviously, appropriately leveled). This is a somewhat different problem from a lot of other bosses. Also, instead of a strong melee attack, I suggest a strong missile attack--give him reduced movement energy like the cat lord and respectable speed, and make him fight more like a quickling--shoot missiles and retreat, and a more modest melee attack. He could even fire that funky bow that slows you.
As far as new items go:
-Scattered around the swamp area are herb bushes. Rather than producing regular herbs, they produce the following:
--Swamp creeper/Blood thorn: Recovers/Diminishes PP by 5/10/30 for B/U/C when eaten.
--Colossus vine/Swamp carrot: Colossus vines are very heavy and train St when 'u'sed; swamp carrots train Pe when eaten.
--Ambrosia flower: Reduces corruption by 10/30/50 for B/U/C when eaten; lowers max HP by 5/3/2 points per use.
--Gambler's folly: Trains a random stat when eaten; corrupts by 50/30/10.
As well, gardeners may collect "enchanted herb seeds" that allow these plants to be grown in other (wilderness only!) areas.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.