I think sticking to hard dungeons with some limitations is a good idea. Apart from Avatar's suggestion I would like to see Hexx Ruins Purist and miser not using Assassin, Paladin or any of the special classes. Don't know if that's a challenge, so add vicious token if you think it is too easy.
I like the idea of hard dungeons plus limitations, though since we already did Hex ruins I doubt we'll go back there for awhile.
My next suggestion would be something along the lines of "Naga City, must be lvl 1 going down to the arena" but I don't know if that is possible. I wouldn't mind giving it a try though. I think my class/race of choice would have to be Orc Warlord, but that certainly wouldn't be part of the restriction.