I am currently planning a campaign, or, rather two campaigns. The one I'm discussing in this post, however is intended to be an experiment in old school styles of play. It will be a sandbox campaign in structure, focusing on a megadungeon, "Castle Acheron", but also including the area surrounding that edifice.
Castle Acheron was the domain of a powerful necromancer, known by the title of "Gravelord". The Gravelord seized control of the castle due to it's presence at a convergence of laylines. He then proceeded to command his dwarven servants to create a series of catacombs, which served as not only a place for his dark laboratories where he experimented with bridging the gap between this life and the next, storage places for his powerful magical treasures, and housing for his minions and undead creations, but also he sought to use the construction to in fact weaken the barriers between the material world and the dread realm of Hades. This is why I chose the name "Castle Acheron" after the river that serves as the border of the land of the dead. In fact, it's likely that the players will visit the realm of the dead, by finding a portal to Hades in one of the lower layers.
As for the upper layers I'm pretty sure that dwarves will be a common threat. My dwarves are quite different then other dwarves, which I will elaborate on in my next post. Common dungeon threats such as goblins and kobold may not appear, which again I will address in my next post. Undead of various types are likely to be spread throughout the entire dungeon, in varying degrees depending on the layer of course. There will certainly be various kinds of vermin. Traps and tricks of various kinds will be a major feature. The lair of a powerful necromancer is a dangerous place. As you go lower, the influence of the land of the dead will get stronger. The idea that the region beneath the surface of the world is closer to the realm of the dead is true everywhere, in fact my world's equivalent to the Underdark is called the Underworld like the land of the dead it's self because of this fact. Castle Acheron is built to take advantage of this.